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A sequence ${a: {\bf Z} \rightarrow {\bf C}}$ of complex numbers is said to be quasiperiodic if it is of the form

$\displaystyle a(n) = F( \alpha_1 n \hbox{ mod } 1, \dots, \alpha_k n \hbox{ mod } 1)$

for some real numbers ${\alpha_1,\dots,\alpha_k}$ and continuous function ${F: ({\bf R}/{\bf Z})^k \rightarrow {\bf C}}$. For instance, linear phases such as ${n \mapsto e(\alpha n + \beta)}$ (where ${e(\theta) := e^{2\pi i \theta}}$) are examples of quasiperiodic sequences; the top order coefficient ${\alpha}$ (modulo ${1}$) can be viewed as a “frequency” of the integers, and an element of the Pontryagin dual ${\hat {\bf Z} \equiv {\bf R}/{\bf Z}}$ of the integers. Any periodic sequence is also quasiperiodic (taking ${k=1}$ and ${\alpha_1}$ to be the reciprocal of the period). A sequence is said to be almost periodic if it is the uniform limit of quasiperiodic sequences. For instance any Fourier series of the form

$\displaystyle a(n) = \sum_{j=1}^\infty c_j e(\alpha_j n)$

with ${\alpha_1,\alpha_2,\dots}$ real numbers and ${c_1,c_2,\dots}$ an absolutely summable sequence of complex coefficients, will be almost periodic.

These sequences arise in various “complexity one” problems in arithmetic combinatorics and ergodic theory. For instance, if ${(X, \mu, T)}$ is a measure-preserving system – a probability space ${(X,\mu)}$ equipped with a measure-preserving shift, and ${f_1,f_2 \in L^\infty(X)}$ are bounded measurable functions, then the correlation sequence

$\displaystyle a(n) := \int_X f_1(x) f_2(T^n x)\ d\mu(x)$

can be shown to be an almost periodic sequence, plus an error term ${b_n}$ which is “null” in the sense that it has vanishing uniform density:

$\displaystyle \sup_N \frac{1}{M} \sum_{n=N+1}^{N+M} |b_n| \rightarrow 0 \hbox{ as } M \rightarrow \infty.$

This can be established in a number of ways, for instance by writing ${a(n)}$ as the Fourier coefficients of the spectral measure of the shift ${T}$ with respect to the functions ${f_1,f_2}$, and then decomposing that measure into pure point and continuous components.

In the last two decades or so, it has become clear that there are natural higher order versions of these concepts, in which linear polynomials such as ${\alpha n + \beta}$ are replaced with higher degree counterparts. The most obvious candidates for these counterparts would be the polynomials ${\alpha_d n^d + \dots + \alpha_0}$, but this turns out to not be a complete set of higher degree objects needed for the theory. Instead, the higher order versions of quasiperiodic and almost periodic sequences are now known as basic nilsequences and nilsequences respectively, while the higher order version of a linear phase is a nilcharacter; each nilcharacter then has a symbol that is a higher order generalisation of the concept of a frequency (and the collection of all symbols forms a group that can be viewed as a higher order version of the Pontryagin dual of ${{\bf Z}}$). The theory of these objects is spread out in the literature across a number of papers; in particular, the theory of nilcharacters is mostly developed in Appendix E of this 116-page paper of Ben Green, Tamar Ziegler, and myself, and is furthermore written using nonstandard analysis and treating the more general setting of higher dimensional sequences. I therefore decided to rewrite some of that material in this blog post, in the simpler context of the qualitative asymptotic theory of one-dimensional nilsequences and nilcharacters rather than the quantitative single-scale theory that is needed for combinatorial applications (and which necessitated the use of nonstandard analysis in the previous paper).

For technical reasons (having to do with the non-trivial topological structure on nilmanifolds), it will be convenient to work with vector-valued sequences, that take values in a finite-dimensional complex vector space ${{\bf C}^m}$ rather than in ${{\bf C}}$. By doing so, the space of sequences is now, technically, no longer a ring, as the operations of addition and multiplication on vector-valued sequences become ill-defined. However, we can still take complex conjugates of a sequence, and add sequences taking values in the same vector space ${{\bf C}^m}$, and for sequences taking values in different vector spaces ${{\bf C}^m}$, ${{\bf C}^{m'}}$, we may utilise the tensor product ${\otimes: {\bf C}^m \times {\bf C}^{m'} \rightarrow {\bf C}^{mm'}}$, which we will normalise by defining

$\displaystyle (z_1,\dots,z_m) \otimes (w_1,\dots,w_{m'}) = (z_1 w_1, \dots, z_1 w_{m'}, \dots, z_m w_1, \dots, z_m w_{m'} ).$

This product is associative and bilinear, and also commutative up to permutation of the indices. It also interacts well with the Hermitian norm

$\displaystyle \| (z_1,\dots,z_m) \| := \sqrt{|z_1|^2 + \dots + |z_m|^2}$

since we have ${\|z \otimes w\| = \|z\| \|w\|}$.

The traditional definition of a basic nilsequence (as defined for instance by Bergelson, Host, and Kra) is as follows:

Definition 1 (Basic nilsequence, first definition) A nilmanifold of step at most ${d}$ is a quotient ${G/\Gamma}$, where ${G}$ is a connected, simply connected nilpotent Lie group of step at most ${d}$ (thus, all ${d+1}$-fold commutators vanish) and ${\Gamma}$ is a discrete cocompact lattice in ${G}$. A basic nilsequence of degree at most ${d}$ is a sequence of the form ${n \mapsto F(g^n g_0 \Gamma)}$, where ${g_0 \Gamma \in G/\Gamma}$, ${g \in G}$, and ${F: G/\Gamma \rightarrow {\bf C}^m}$ is a continuous function.

For instance, it is not difficult using this definition to show that a sequence is a basic nilsequence of degree at most ${1}$ if and only if it is quasiperiodic. The requirement that ${G}$ be simply connected can be easily removed if desired by passing to a universal cover, but it is technically convenient to assume it (among other things, it allows for a well-defined logarithm map that obeys the Baker-Campbell-Hausdorff formula). When one wishes to perform a more quantitative analysis of nilsequences (particularly when working on a “single scale”. sich as on a single long interval ${\{ N+1, \dots, N+M\}}$), it is common to impose additional regularity conditions on the function ${F}$, such as Lipschitz continuity or smoothness, but ordinary continuity will suffice for the qualitative discussion in this blog post.

Nowadays, and particularly when one needs to understand the “single-scale” equidistribution properties of nilsequences, it is more convenient (as is for instance done in this ICM paper of Green) to use an alternate definition of a nilsequence as follows.

Definition 2 Let ${d \geq 0}$. A filtered group of degree at most ${d}$ is a group ${G}$ together with a sequence ${G_\bullet = (G_0,G_1,G_2,\dots)}$ of subgroups ${G \geq G_0 \geq G_1 \geq \dots}$ with ${G_{d+1}=\{\hbox{id}\}}$ and ${[G_i,G_j] \subset G_{i+j}}$ for ${i,j \geq 0}$. A polynomial sequence ${g: {\bf Z} \rightarrow G}$ into a filtered group is a function such that ${\partial_{h_i} \dots \partial_{h_1} g(n) \in G_i}$ for all ${i \geq 0}$ and ${n,h_1,\dots,h_i \in{\bf Z}}$, where ${\partial_h g(n) := g(n+h) g(n)^{-1}}$ is the difference operator. A filtered nilmanifold of degree at most ${s}$ is a quotient ${G/\Gamma}$, where ${G}$ is a filtered group of degree at most ${s}$ such that ${G}$ and all of the subgroups ${G_i}$ are connected, simply connected nilpotent filtered Lie group, and ${\Gamma}$ is a discrete cocompact subgroup of ${G}$ such that ${\Gamma_i := \Gamma \cap G_i}$ is a discrete cocompact subgroup of ${G_i}$. A basic nilsequence of degree at most ${d}$ is a sequence of the form ${n \mapsto F(g(n))}$, where ${g: {\bf Z} \rightarrow G}$ is a polynomial sequence, ${G/\Gamma}$ is a filtered nilmanifold of degree at most ${d}$, and ${F: G \rightarrow {\bf C}^m}$ is a continuous function which is ${\Gamma}$-automorphic, in the sense that ${F(g \gamma) = F(g)}$ for all ${g \in G}$ and ${\gamma \in \Gamma}$.

One can easily identify a ${\Gamma}$-automorphic function on ${G}$ with a function on ${G/\Gamma}$, but there are some (very minor) advantages to working on the group ${G}$ instead of the quotient ${G/\Gamma}$, as it becomes slightly easier to modify the automorphy group ${\Gamma}$ when needed. (But because the action of ${\Gamma}$ on ${G}$ is free, one can pass from ${\Gamma}$-automorphic functions on ${G}$ to functions on ${G/\Gamma}$ with very little difficulty.) The main reason to work with polynomial sequences ${n \mapsto g(n)}$ rather than geometric progressions ${n \mapsto g^n g_0 \Gamma}$ is that they form a group, a fact essentially established by by Lazard and Leibman; see Corollary B.4 of this paper of Green, Ziegler, and myself for a proof in the filtered group setting.

It is easy to see that any sequence that is a basic nilsequence of degree at most ${d}$ in the sense of the first definition, is also a basic nilsequence of degree at most ${d}$ in the second definition, since a nilmanifold of degree at most ${d}$ can be filtered using the lower central series, and any linear sequence ${n \mapsto g^n g_0}$ will be a polynomial sequence with respect to that filtration. The converse implication is a little trickier, but still not too hard to show: see Appendix C of this paper of Ben Green, Tamar Ziegler, and myself. There are two key examples of basic nilsequences to keep in mind. The first are the polynomially quasiperiodic sequences

$\displaystyle a(n) = F( P_1(n), \dots, P_k(n) ),$

where ${P_1,\dots,P_k: {\bf Z} \rightarrow {\bf R}}$ are polynomials of degree at most ${d}$, and ${F: {\bf R}^k \rightarrow {\bf C}^m}$ is a ${{\bf Z}^k}$-automorphic (i.e., ${{\bf Z}^k}$-periodic) continuous function. The map ${P: {\bf Z} \rightarrow {\bf R}^k}$ defined by ${P(n) := (P_1(n),\dots,P_k(n))}$ is a polynomial map of degree at most ${d}$, if one filters ${{\bf R}^k}$ by defining ${({\bf R}^k)_i}$ to equal ${{\bf R}^k}$ when ${i \leq d}$, and ${\{0\}}$ for ${i > d}$. The torus ${{\bf R}^k/{\bf Z}^k}$ then becomes a filtered nilmanifold of degree at most ${d}$, and ${a(n)}$ is thus a basic nilsequence of degree at most ${d}$ as per the second definition. It is also possible explicitly describe ${a_n}$ as a basic nilsequence of degree at most ${d}$ as per the first definition, for instance (in the ${k=1}$ case) by taking ${G}$ to be the space of upper triangular unipotent ${d+1 \times d+1}$ real matrices, and ${\Gamma}$ the subgroup with integer coefficients; we leave the details to the interested reader.

The other key example is a sequence of the form

$\displaystyle a(n) = F( \alpha n, \{ \alpha n \} \beta n )$

where ${\alpha,\beta}$ are real numbers, ${\{ \alpha n \} = \alpha n - \lfloor \alpha n \rfloor}$ denotes the fractional part of ${\alpha n}$, and and ${F: {\bf R}^2 \rightarrow {\bf C}^m}$ is a ${{\bf Z}^2}$-automorphic continuous function that vanishes in a neighbourhood of ${{\bf Z} \times {\bf R}}$. To describe this as a nilsequence, we use the nilpotent connected, simply connected degree ${2}$, Heisenberg group

$\displaystyle G := \begin{pmatrix} 1 & {\bf R} & {\bf R} \\ 0 & 1 & {\bf R} \\ 0 & 0 & 1 \end{pmatrix}$

with the lower central series filtration ${G_0=G_1=G}$, ${G_2= [G,G] = \begin{pmatrix} 1 &0 & {\bf R} \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}}$, and ${G_i = \{ \mathrm{id} \}}$ for ${i > 2}$, ${\Gamma}$ to be the discrete compact subgroup

$\displaystyle \Gamma := \begin{pmatrix} 1 & {\bf Z} & {\bf Z} \\ 0 & 1 & {\bf Z} \\ 0 & 0 & 1 \end{pmatrix},$

${g: {\bf Z} \rightarrow G}$ to be the polynomial sequence

$\displaystyle g(n) := \begin{pmatrix} 1 & \beta n & \alpha \beta n^2 \\ 0 & 1 & \alpha n \\ 0 & 0 & 1 \end{pmatrix}$

and ${\tilde F: G \rightarrow {\bf C}^m}$ to be the ${\Gamma}$-automorphic function

$\displaystyle \tilde F( \begin{pmatrix} 1 & x & y \\ 0 & 1 & z \\ 0 & 0 & 1 \end{pmatrix} ) = F( \{ z \}, y - \lfloor z \rfloor x );$

one easily verifies that this function is indeed ${\Gamma}$-automorphic, and it is continuous thanks to the vanishing properties of ${F}$. Also we have ${a(n) = \tilde F(g(n))}$, so ${a}$ is a basic nilsequence of degree at most ${2}$. One can concoct similar examples with ${\{ \alpha n \} \beta n}$ replaced by other “bracket polynomials” of ${n}$; for instance

$\displaystyle a(n) = F( \alpha n, \{ \alpha n - \frac{1}{2} \} \beta n )$

will be a basic nilsequence if ${F}$ now vanishes in a neighbourhood of ${(\frac{1}{2}+{\bf Z}) \times {\bf R}}$ rather than ${{\bf Z} \times {\bf R}}$. See this paper of Bergelson and Leibman for more discussion of bracket polynomials (also known as generalised polynomials) and their relationship to nilsequences.

A nilsequence of degree at most ${d}$ is defined to be a sequence that is the uniform limit of basic nilsequences of degree at most ${d}$. Thus for instance a sequence is a nilsequence of degree at most ${1}$ if and only if it is almost periodic, while a sequence is a nilsequence of degree at most ${0}$ if and only if it is constant. Such objects arise in higher order recurrence: for instance, if ${h_0,\dots,h_d}$ are integers, ${(X,\mu,T)}$ is a measure-preserving system, and ${f_0,\dots,f_d \in L^\infty(X)}$, then it was shown by Leibman that the sequence

$\displaystyle n \mapsto \int_X f_0(T^{h_0 n} x) \dots f_d(T^{h_d n} x)\ d\mu(x)$

is equal to a nilsequence of degree at most ${d}$, plus a null sequence. (The special case when the measure-preserving system was ergodic and ${h_i = i}$ for ${i=0,\dots,d}$ was previously established by Bergelson, Host, and Kra.) Nilsequences also arise in the inverse theory of the Gowers uniformity norms, as discussed for instance in this previous post.

It is easy to see that a sequence ${a: {\bf Z} \rightarrow {\bf C}^m}$ is a basic nilsequence of degree at most ${d}$ if and only if each of its ${m}$ components are. The scalar basic nilsequences ${a: {\bf Z} \rightarrow {\bf C}}$ of degree ${d}$ are easily seen to form a ${*}$-algebra (that is to say, they are a complex vector space closed under pointwise multiplication and complex conjugation), which implies similarly that vector-valued basic nilsequences ${a: {\bf Z} \rightarrow {\bf C}^m}$ of degree at most ${d}$ form a complex vector space closed under complex conjugation for each ${m}$, and that the tensor product of any two basic nilsequences of degree at most ${d}$ is another basic nilsequence of degree at most ${d}$. Similarly with “basic nilsequence” replaced by “nilsequence” throughout.

Now we turn to the notion of a nilcharacter, as defined in this paper of Ben Green, Tamar Ziegler, and myself:

Definition 3 (Nilcharacters) Let ${d \geq 1}$. A sub-nilcharacter of degree ${d}$ is a basic nilsequence ${\chi: n \mapsto F(g(n))}$ of degree at most ${d}$, such that ${F}$ obeys the additional modulation property

$\displaystyle F( g_d g ) = e( \xi \cdot g_d ) F(g) \ \ \ \ \ (1)$

for all ${g \in G}$ and ${g_d \in G_d}$, where ${\xi: G_d \rightarrow {\bf R}}$ is a continuous homomorphism ${g_d \mapsto \xi \cdot g_d}$. (Note from (1) and ${\Gamma}$-automorphy that unless ${F}$ vanishes identically, ${\xi}$ must map ${\Gamma_d}$ to ${{\bf Z}}$, thus without loss of generality one can view ${\xi}$ as an element of the Pontryagial dual of the torus ${G_d / \Gamma_d}$.) If in addition one has ${\|F(g)\|=1}$ for all ${g \in G}$, we call ${\chi}$ a nilcharacter of degree ${d \geq 1}$.

In the degree one case ${d=1}$, the only sub-nilcharacters are of the form ${\chi(n) = e(\alpha n)}$ for some vector ${c \in {\bf C}^m}$ and ${\alpha \in {\bf R}}$, and this is a nilcharacter if ${c}$ is a unit vector. Similarly, in higher degree, any sequence of the form ${\chi(n) = c e(P(n))}$, where ${c \in {\bf C}^m}$ is a vector and ${P: {\bf Z} \rightarrow {\bf R}}$ is a polynomial of degree at most ${d}$, is a sub-nilcharacter of degree ${d}$, and a character if ${c}$ is a unit vector. A nilsequence of degree at most ${d-1}$ is automatically a sub-nilcharacter of degree ${d}$, and a nilcharacter if it is of magnitude ${1}$. A further example of a nilcharacter is provided by the two-dimensional sequence ${\chi: {\bf Z} \rightarrow {\bf C}^2}$ defined by

$\displaystyle \chi(n) := ( F_0( \alpha n ) e( \{ \alpha n \} \beta n ), F_{1/2}( \alpha n ) e( \{ \alpha n - \frac{1}{2} \} \beta n ) ) \ \ \ \ \ (2)$

where ${F_0, F_{1/2}: {\bf R} \rightarrow {\bf C}}$ are continuous, ${{\bf Z}}$-automorphic functions that vanish on a neighbourhood of ${{\bf Z}}$ and ${\frac{1}{2}+{\bf Z}}$ respectively, and which form a partition of unity in the sense that

$\displaystyle |F_0(x)|^2 + |F_{1/2}(x)|^2 = 1$

for all ${x \in {\bf R}}$. Note that one needs both ${F_0}$ and ${F_{1/2}}$ to be not identically zero in order for all these conditions to be satisfied; it turns out (for topological reasons) that there is no scalar nilcharacter that is “equivalent” to this nilcharacter in a sense to be defined shortly. In some literature, one works exclusively with sub-nilcharacters rather than nilcharacters, however the former space contains zero-divisors, which is a little annoying technically. Nevertheless, both nilcharacters and sub-nilcharacters generate the same set of “symbols” as we shall see later.

We claim that every degree ${d}$ sub-nilcharacter ${f: {\bf Z} \rightarrow {\bf C}^m}$ can be expressed in the form ${f = c \chi}$, where ${\chi: {\bf Z} \rightarrow {\bf C}^{m'}}$ is a degree ${d}$ nilcharacter, and ${c: {\bf C}^{m'} \rightarrow {\bf C}^m}$ is a linear transformation. Indeed, by scaling we may assume ${f(n) = F(g(n))}$ where ${|F| < 1}$ uniformly. Using partitions of unity, one can find further functions ${F_1,\dots,F_m}$ also obeying (1) for the same character ${\xi}$ such that ${|F_1|^2 + \dots + |F_m|^2}$ is non-zero; by dividing out the ${F_1,\dots,F_m}$ by the square root of this quantity, and then multiplying by ${\sqrt{1-|F|^2}}$, we may assume that

$\displaystyle |F|^2 + |F_1|^2 + \dots + |F_m|^2 = 1,$

and then

$\displaystyle \chi(n) := (F(g(n)), F_1(g(n)), \dots, F_m(g(n)))$

becomes a degree ${d}$ nilcharacter that contains ${f(n)}$ amongst its components, giving the claim.

As we shall show below, nilsequences can be approximated uniformly by linear combinations of nilcharacters, in much the same way that quasiperiodic or almost periodic sequences can be approximated uniformly by linear combinations of linear phases. In particular, nilcharacters can be used as “obstructions to uniformity” in the sense of the inverse theory of the Gowers uniformity norms.

The space of degree ${d}$ nilcharacters forms a semigroup under tensor product, with the constant sequence ${1}$ as the identity. One can upgrade this semigroup to an abelian group by quotienting nilcharacters out by equivalence:

Definition 4 Let ${d \geq 1}$. We say that two degree ${d}$ nilcharacters ${\chi: {\bf Z} \rightarrow {\bf C}^m}$, ${\chi': {\bf Z} \rightarrow {\bf C}^{m'}}$ are equivalent if ${\chi \otimes \overline{\chi'}: {\bf Z} \rightarrow {\bf C}^{mm'}}$ is equal (as a sequence) to a basic nilsequence of degree at most ${d-1}$. (We will later show that this is indeed an equivalence relation.) The equivalence class ${[\chi]_{\mathrm{Symb}^d({\bf Z})}}$ of such a nilcharacter will be called the symbol of that nilcharacter (in analogy to the symbol of a differential or pseudodifferential operator), and the collection of such symbols will be denoted ${\mathrm{Symb}^d({\bf Z})}$.

As we shall see below the fold, ${\mathrm{Symb}^d({\bf Z})}$ has the structure of an abelian group, and enjoys some nice “symbol calculus” properties; also, one can view symbols as precisely describing the obstruction to equidistribution for nilsequences. For ${d=1}$, the group is isomorphic to the Ponytragin dual ${\hat {\bf Z} = {\bf R}/{\bf Z}}$ of the integers, and ${\mathrm{Symb}^d({\bf Z})}$ for ${d > 1}$ should be viewed as higher order generalisations of this Pontryagin dual. In principle, this group can be explicitly described for all ${d}$, but the theory rapidly gets complicated as ${d}$ increases (much as the classification of nilpotent Lie groups or Lie algebras of step ${d}$ rapidly gets complicated even for medium-sized ${d}$ such as ${d=3}$ or ${d=4}$). We will give an explicit description of the ${d=2}$ case here. There is however one nice (and non-trivial) feature of ${\mathrm{Symb}^d({\bf Z})}$ for ${d \geq 2}$ – it is not just an abelian group, but is in fact a vector space over the rationals ${{\bf Q}}$!

How many groups of order four are there? Technically, there are an enormous number, so much so, in fact, that the class of groups of order four is not even a set, but merely a proper class. This is because any four objects ${a,b,c,d}$ can be turned into a group ${\{a,b,c,d\}}$ by designating one of the four objects, say ${a}$, to be the group identity, and imposing a suitable multiplication table (and inversion law) on the four elements in a manner that obeys the usual group axioms. Since all sets are themselves objects, the class of four-element groups is thus at least as large as the class of all sets, which by Russell’s paradox is known not to itself be a set (assuming the usual ZFC axioms of set theory).

A much better question is to ask how many groups of order four there are up to isomorphism, counting each isomorphism class of groups exactly once. Now, as one learns in undergraduate group theory classes, the answer is just “two”: the cyclic group ${C_4}$ and the Klein four-group ${C_2 \times C_2}$.

More generally, given a class of objects ${X}$ and some equivalence relation ${\sim}$ on ${X}$ (which one should interpret as describing the property of two objects in ${X}$ being “isomorphic”), one can consider the number ${|X / \sim|}$ of objects in ${X}$ “up to isomorphism”, which is simply the cardinality of the collection ${X/\sim}$ of equivalence classes ${[x]:=\{y\in X:x \sim {}y \}}$ of ${X}$. In the case where ${X}$ is finite, one can express this cardinality by the formula

$\displaystyle |X/\sim| = \sum_{x \in X} \frac{1}{|[x]|}, \ \ \ \ \ (1)$

thus one counts elements in ${X}$, weighted by the reciprocal of the number of objects they are isomorphic to.

Example 1 Let ${X}$ be the five-element set ${\{-2,-1,0,1,2\}}$ of integers between ${-2}$ and ${2}$. Let us say that two elements ${x, y}$ of ${X}$ are isomorphic if they have the same magnitude: ${x \sim y \iff |x| = |y|}$. Then the quotient space ${X/\sim}$ consists of just three equivalence classes: ${\{-2,2\} = [2] = [-2]}$, ${\{-1,1\} = [-1] = [1]}$, and ${\{0\} = [0]}$. Thus there are three objects in ${X}$ up to isomorphism, and the identity (1) is then just

$\displaystyle 3 = \frac{1}{2} + \frac{1}{2} + 1 + \frac{1}{2} + \frac{1}{2}.$

Thus, to count elements in ${X}$ up to equivalence, the elements ${-2,-1,1,2}$ are given a weight of ${1/2}$ because they are each isomorphic to two elements in ${X}$, while the element ${0}$ is given a weight of ${1}$ because it is isomorphic to just one element in ${X}$ (namely, itself).

Given a finite probability set ${X}$, there is also a natural probability distribution on ${X}$, namely the uniform distribution, according to which a random variable ${\mathbf{x} \in X}$ is set equal to any given element ${x}$ of ${X}$ with probability ${\frac{1}{|X|}}$:

$\displaystyle {\bf P}( \mathbf{x} = x ) = \frac{1}{|X|}.$

Given a notion ${\sim}$ of isomorphism on ${X}$, one can then define the random equivalence class ${[\mathbf{x}] \in X/\sim}$ that the random element ${\mathbf{x}}$ belongs to. But if the isomorphism classes are unequal in size, we now encounter a biasing effect: even if ${\mathbf{x}}$ was drawn uniformly from ${X}$, the equivalence class ${[\mathbf{x}]}$ need not be uniformly distributed in ${X/\sim}$. For instance, in the above example, if ${\mathbf{x}}$ was drawn uniformly from ${\{-2,-1,0,1,2\}}$, then the equivalence class ${[\mathbf{x}]}$ will not be uniformly distributed in the three-element space ${X/\sim}$, because it has a ${2/5}$ probability of being equal to the class ${\{-2,2\}}$ or to the class ${\{-1,1\}}$, and only a ${1/5}$ probability of being equal to the class ${\{0\}}$.

However, it is possible to remove this bias by changing the weighting in (1), and thus changing the notion of what cardinality means. To do this, we generalise the previous situation. Instead of considering sets ${X}$ with an equivalence relation ${\sim}$ to capture the notion of isomorphism, we instead consider groupoids, which are sets ${X}$ in which there are allowed to be multiple isomorphisms between elements in ${X}$ (and in particular, there are allowed to be multiple automorphisms from an element to itself). More precisely:

Definition 2 A groupoid is a set (or proper class) ${X}$, together with a (possibly empty) collection ${\mathrm{Iso}(x \rightarrow y)}$ of “isomorphisms” between any pair ${x,y}$ of elements of ${X}$, and a composition map ${f, g \mapsto g \circ f}$ from isomorphisms ${f \in \mathrm{Iso}(x \rightarrow y)}$, ${g \in \mathrm{Iso}(y \rightarrow z)}$ to isomorphisms in ${\mathrm{Iso}(x \rightarrow z)}$ for every ${x,y,z \in X}$, obeying the following group-like axioms:

• (Identity) For every ${x \in X}$, there is an identity isomorphism ${\mathrm{id}_x \in \mathrm{Iso}(x \rightarrow x)}$, such that ${f \circ \mathrm{id}_x = \mathrm{id}_y \circ f = f}$ for all ${f \in \mathrm{Iso}(x \rightarrow y)}$ and ${x,y \in X}$.
• (Associativity) If ${f \in \mathrm{Iso}(x \rightarrow y)}$, ${g \in \mathrm{Iso}(y \rightarrow z)}$, and ${h \in \mathrm{Iso}(z \rightarrow w)}$ for some ${x,y,z,w \in X}$, then ${h \circ (g \circ f) = (h \circ g) \circ f}$.
• (Inverse) If ${f \in \mathrm{Iso}(x \rightarrow y)}$ for some ${x,y \in X}$, then there exists an inverse isomorphism ${f^{-1} \in \mathrm{Iso}(y \rightarrow x)}$ such that ${f \circ f^{-1} = \mathrm{id}_y}$ and ${f^{-1} \circ f = \mathrm{id}_x}$.

We say that two elements ${x,y}$ of a groupoid are isomorphic, and write ${x \sim y}$, if there is at least one isomorphism from ${x}$ to ${y}$.

Example 3 Any category gives a groupoid by taking ${X}$ to be the set (or class) of objects, and ${\mathrm{Iso}(x \rightarrow y)}$ to be the collection of invertible morphisms from ${x}$ to ${y}$. For instance, in the category ${\mathbf{Set}}$ of sets, ${\mathrm{Iso}(x \rightarrow y)}$ would be the collection of bijections from ${x}$ to ${y}$; in the category ${\mathbf{Vec}/k}$ of linear vector spaces over some given base field ${k}$, ${\mathrm{Iso}(x \rightarrow y)}$ would be the collection of invertible linear transformations from ${x}$ to ${y}$; and so forth.

Every set ${X}$ equipped with an equivalence relation ${\sim}$ can be turned into a groupoid by assigning precisely one isomorphism ${\iota_{x \rightarrow y}}$ from ${x}$ to ${y}$ for any pair ${x,y \in X}$ with ${x \sim y}$, and no isomorphisms from ${x}$ to ${y}$ when ${x \not \sim y}$, with the groupoid operations of identity, composition, and inverse defined in the only way possible consistent with the axioms. We will call this the simply connected groupoid associated with this equivalence relation. For instance, with ${X = \{-2,-1,0,1,2\}}$ as above, if we turn ${X}$ into a simply connected groupoid, there will be precisely one isomorphism from ${2}$ to ${-2}$, and also precisely one isomorphism from ${2}$ to ${2}$, but no isomorphisms from ${2}$ to ${-1}$, ${0}$, or ${1}$.

However, one can also form multiply-connected groupoids in which there can be multiple isomorphisms from one element of ${X}$ to another. For instance, one can view ${X = \{-2,-1,0,1,2\}}$ as a space that is acted on by multiplication by the two-element group ${\{-1,+1\}}$. This gives rise to two types of isomorphisms, an identity isomorphism ${(+1)_x}$ from ${x}$ to ${x}$ for each ${x \in X}$, and a negation isomorphism ${(-1)_x}$ from ${x}$ to ${-x}$ for each ${x \in X}$; in particular, there are two automorphisms of ${0}$ (i.e., isomorphisms from ${0}$ to itself), namely ${(+1)_0}$ and ${(-1)_0}$, whereas the other four elements of ${X}$ only have a single automorphism (the identity isomorphism). One defines composition, identity, and inverse in this groupoid in the obvious fashion (using the group law of the two-element group ${\{-1,+1\}}$); for instance, we have ${(-1)_{-2} \circ (-1)_2 = (+1)_2}$.

For a finite multiply-connected groupoid, it turns out that the natural notion of “cardinality” (or as I prefer to call it, “cardinality up to isomorphism”) is given by the variant

$\displaystyle \sum_{x \in X} \frac{1}{|\{ f: f \in \mathrm{Iso}(x \rightarrow y) \hbox{ for some } y\}|}$

of (1). That is to say, in the multiply connected case, the denominator is no longer the number of objects ${y}$ isomorphic to ${x}$, but rather the number of isomorphisms from ${x}$ to other objects ${y}$. Grouping together all summands coming from a single equivalence class ${[x]}$ in ${X/\sim}$, we can also write this expression as

$\displaystyle \sum_{[x] \in X/\sim} \frac{1}{|\mathrm{Aut}(x)|} \ \ \ \ \ (2)$

where ${\mathrm{Aut}(x) := \mathrm{Iso}(x \rightarrow x)}$ is the automorphism group of ${x}$, that is to say the group of isomorphisms from ${x}$ to itself. (Note that if ${x,x'}$ belong to the same equivalence class ${[x]}$, then the two groups ${\mathrm{Aut}(x)}$ and ${\mathrm{Aut}(x')}$ will be isomorphic and thus have the same cardinality, and so the above expression is well-defined.

For instance, if we take ${X}$ to be the simply connected groupoid on ${\{-2,-1,0,1,2\}}$, then the number of elements of ${X}$ up to isomorphism is

$\displaystyle \frac{1}{2} + \frac{1}{2} + 1 + \frac{1}{2} + \frac{1}{2} = 1 + 1 + 1 = 3$

exactly as before. If however we take the multiply connected groupoid on ${\{-2,-1,0,1,2\}}$, in which ${0}$ has two automorphisms, the number of elements of ${X}$ up to isomorphism is now the smaller quantity

$\displaystyle \frac{1}{2} + \frac{1}{2} + \frac{1}{2} + \frac{1}{2} + \frac{1}{2} = 1 + \frac{1}{2} + 1 = \frac{5}{2};$

the equivalence class ${[0]}$ is now counted with weight ${1/2}$ rather than ${1}$ due to the two automorphisms on ${0}$. Geometrically, one can think of this groupoid as being formed by taking the five-element set ${\{-2,-1,0,1,2\}}$, and “folding it in half” around the fixed point ${0}$, giving rise to two “full” quotient points ${[1], [2]}$ and one “half” point ${[0]}$. More generally, given a finite group ${G}$ acting on a finite set ${X}$, and forming the associated multiply connected groupoid, the cardinality up to isomorphism of this groupoid will be ${|X|/|G|}$, since each element ${x}$ of ${X}$ will have ${|G|}$ isomorphisms on it (whether they be to the same element ${x}$, or to other elements of ${X}$).

The definition (2) can also make sense for some infinite groupoids; to my knowledge this was first explicitly done in this paper of Baez and Dolan. Consider for instance the category ${\mathbf{FinSet}}$ of finite sets, with isomorphisms given by bijections as in Example 3. Every finite set is isomorphic to ${\{1,\dots,n\}}$ for some natural number ${n}$, so the equivalence classes of ${\mathbf{FinSet}}$ may be indexed by the natural numbers. The automorphism group ${S_n}$ of ${\{1,\dots,n\}}$ has order ${n!}$, so the cardinality of ${\mathbf{FinSet}}$ up to isomorphism is

$\displaystyle \sum_{n=0}^\infty \frac{1}{n!} = e.$

(This fact is sometimes loosely stated as “the number of finite sets is ${e}$“, but I view this statement as somewhat misleading if the qualifier “up to isomorphism” is not added.) Similarly, when one allows for multiple isomorphisms from a group to itself, the number of groups of order four up to isomorphism is now

$\displaystyle \frac{1}{2} + \frac{1}{6} = \frac{2}{3}$

because the cyclic group ${C_4}$ has two automorphisms, whereas the Klein four-group ${C_2 \times C_2}$ has six.

In the case that the cardinality of a groupoid ${X}$ up to isomorphism is finite and non-empty, one can now define the notion of a random isomorphism class ${[\mathbf{x}]}$ in ${X/\sim}$ drawn “uniformly up to isomorphism”, by requiring the probability of attaining any given isomorphism class ${[x]}$ to be

$\displaystyle {\mathbf P}([\mathbf{x}] = [x]) = \frac{1 / |\mathrm{Aut}(x)|}{\sum_{[y] \in X/\sim} 1/|\mathrm{Aut}(y)|},$

thus the probability of being isomorphic to a given element ${x}$ will be inversely proportional to the number of automorphisms that ${x}$ has. For instance, if we take ${X}$ to be the set ${\{-2,-1,0,1,2\}}$ with the simply connected groupoid, ${[\mathbf{x}]}$ will be drawn uniformly from the three available equivalence classes ${[0], [1], [2]}$, with a ${1/3}$ probability of attaining each; but if instead one uses the multiply connected groupoid coming from the action of ${\{-1,+1\}}$, and draws ${[\mathbf{x}]}$ uniformly up to isomorphism, then ${[1]}$ and ${[2]}$ will now be selected with probability ${2/5}$ each, and ${[0]}$ will be selected with probability ${1/5}$. Thus this distribution has accounted for the bias mentioned previously: if a finite group ${G}$ acts on a finite space ${X}$, and ${\mathbf{x}}$ is drawn uniformly from ${X}$, then ${[\mathbf{x}]}$ now still be drawn uniformly up to isomorphism from ${X/G}$, if we use the multiply connected groupoid coming from the ${G}$ action, rather than the simply connected groupoid coming from just the ${G}$-orbit structure on ${X}$.

Using the groupoid of finite sets, we see that a finite set chosen uniformly up to isomorphism will have a cardinality that is distributed according to the Poisson distribution of parameter ${1}$, that is to say it will be of cardinality ${n}$ with probability ${\frac{e^{-1}}{n!}}$.

One important source of groupoids are the fundamental groupoids ${\pi_1(M)}$ of a manifold ${M}$ (one can also consider more general topological spaces than manifolds, but for simplicity we will restrict this discussion to the manifold case), in which the underlying space is simply ${M}$, and the isomorphisms from ${x}$ to ${y}$ are the equivalence classes of paths from ${x}$ to ${y}$ up to homotopy; in particular, the automorphism group of any given point is just the fundamental group of ${M}$ at that base point. The equivalence class ${[x]}$ of a point in ${M}$ is then the connected component of ${x}$ in ${M}$. The cardinality up to isomorphism of the fundamental groupoid is then

$\displaystyle \sum_{M' \in \pi_0(M)} \frac{1}{|\pi_1(M')|}$

where ${\pi_0(M)}$ is the collection of connected components ${M'}$ of ${M}$, and ${|\pi_1(M')|}$ is the order of the fundamental group of ${M'}$. Thus, simply connected components of ${M}$ count for a full unit of cardinality, whereas multiply connected components (which can be viewed as quotients of their simply connected cover by their fundamental group) will count for a fractional unit of cardinality, inversely to the order of their fundamental group.

This notion of cardinality up to isomorphism of a groupoid behaves well with respect to various basic notions. For instance, suppose one has an ${n}$-fold covering map ${\pi: X \rightarrow Y}$ of one finite groupoid ${Y}$ by another ${X}$. This means that ${\pi}$ is a functor that is surjective, with all preimages of cardinality ${n}$, with the property that given any pair ${y,y'}$ in the base space ${Y}$ and any ${x}$ in the preimage ${\pi^{-1}(\{y\})}$ of ${y}$, every isomorphism ${f \in \mathrm{Iso}(y \rightarrow y')}$ has a unique lift ${\tilde f \in \mathrm{Iso}(x \rightarrow x')}$ from the given initial point ${x}$ (and some ${x'}$ in the preimage of ${y'}$). Then one can check that the cardinality up to isomorphism of ${X}$ is ${n}$ times the cardinality up to isomorphism of ${Y}$, which fits well with the geometric picture of ${X}$ as the ${n}$-fold cover of ${Y}$. (For instance, if one covers a manifold ${M}$ with finite fundamental group by its universal cover, this is a ${|\pi_1(M)|}$-fold cover, the base has cardinality ${1/|\pi_1(M)|}$ up to isomorphism, and the universal cover has cardinality one up to isomorphism.) Related to this, if one draws an equivalence class ${[\mathrm{x}]}$ of ${X}$ uniformly up to isomorphism, then ${\pi([\mathrm{x}])}$ will be an equivalence class of ${Y}$ drawn uniformly up to isomorphism also.

Indeed, one can show that this notion of cardinality up to isomorphism for groupoids is uniquely determined by a small number of axioms such as these (similar to the axioms that determine Euler characteristic); see this blog post of Qiaochu Yuan for details.

The probability distributions on isomorphism classes described by the above recipe seem to arise naturally in many applications. For instance, if one draws a profinite abelian group up to isomorphism at random in this fashion (so that each isomorphism class ${[G]}$ of a profinite abelian group ${G}$ occurs with probability inversely proportional to the number of automorphisms of this group), then the resulting distribution is known as the Cohen-Lenstra distribution, and seems to emerge as the natural asymptotic distribution of many randomly generated profinite abelian groups in number theory and combinatorics, such as the class groups of random quadratic fields; see this previous blog post for more discussion. For a simple combinatorial example, the set of fixed points of a random permutation on ${n}$ elements will have a cardinality that converges in distribution to the Poisson distribution of rate ${1}$ (as discussed in this previous post), thus we see that the fixed points of a large random permutation asymptotically are distributed uniformly up to isomorphism. I’ve been told that this notion of cardinality up to isomorphism is also particularly compatible with stacks (which are a good framework to describe such objects as moduli spaces of algebraic varieties up to isomorphism), though I am not sufficiently acquainted with this theory to say much more than this.