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This post is a continuation of the previous post on sieve theory, which is an ongoing part of the Polymath8 project. As the previous post was getting somewhat full, we are rolling the thread over to the current post.

In this post we will record a new truncation of the elementary Selberg sieve discussed in this previous post (and also analysed in the context of bounded prime gaps by Graham-Goldston-Pintz-Yildirim and Motohashi-Pintz) that was recently worked out by Janos Pintz, who has kindly given permission to share this new idea with the Polymath8 project. This new sieve decouples the {\delta} parameter that was present in our previous analysis of Zhang’s argument into two parameters, a quantity {\delta} that used to measure smoothness in the modulus, but now measures a weaker notion of “dense divisibility” which is what is really needed in the Elliott-Halberstam type estimates, and a second quantity {\delta'} which still measures smoothness but is allowed to be substantially larger than {\delta}. Through this decoupling, it appears that the {\kappa} type losses in the sieve theoretic part of the argument can be almost completely eliminated (they basically decay exponential in {\delta'} and have only mild dependence on {\delta}, whereas the Elliott-Halberstam analysis is sensitive only to {\delta}, allowing one to set {\delta} far smaller than previously by keeping {\delta'} large). This should lead to noticeable gains in the {k_0} quantity in our analysis.

To describe this new truncation we need to review some notation. As in all previous posts (in particular, the first post in this series), we have an asymptotic parameter {x} going off to infinity, and all quantities here are implicitly understood to be allowed to depend on {x} (or to range in a set that depends on {x}) unless they are explicitly declared to be fixed. We use the usual asymptotic notation {O(), o(), \ll} relative to this parameter {x}. To be able to ignore local factors (such as the singular series {{\mathfrak G}}), we also use the “{W}-trick” (as discussed in the first post in this series): we introduce a parameter {w} that grows very slowly with {x}, and set {W := \prod_{p<w} p}.

For any fixed natural number {k_0}, define an admissible {k_0}-tuple to be a fixed tuple {{\mathcal H}} of {k_0} distinct integers which avoids at least one residue class modulo {p} for each prime {p}. Our objective is to obtain the following conjecture {DHL[k_0,2]} for as small a value of the parameter {k_0} as possible:

Conjecture 1 ({DHL[k_0,2]}) Let {{\mathcal H}} be a fixed admissible {k_0}-tuple. Then there exist infinitely many translates {n+{\mathcal H}} of {{\mathcal H}} that contain at least two primes.

The twin prime conjecture asserts that {DHL[k_0,2]} holds for {k_0} as small as {2}, but currently we are only able to establish this result for {k_0 \geq 6329} (see this comment). However, with the new truncated sieve of Pintz described in this post, we expect to be able to lower this threshold {k_0 \geq 6329} somewhat.

In previous posts, we deduced {DHL[k_0,2]} from a technical variant {MPZ[\varpi,\delta]} of the Elliot-Halberstam conjecture for certain choices of parameters {0 < \varpi < 1/4}, {0 < \delta < 1/4+\varpi}. We will use the following formulation of {MPZ[\varpi,\delta]}:

Conjecture 2 ({MPZ[\varpi,\delta]}) Let {{\mathcal H}} be a fixed {k_0}-tuple (not necessarily admissible) for some fixed {k_0 \geq 2}, and let {b\ (W)} be a primitive residue class. Then

\displaystyle  \sum_{q \in {\mathcal S}_I: q< x^{1/2+2\varpi}} \sum_{a \in C(q)} |\Delta_{b,W}(\Lambda; q,a)| = O( x \log^{-A} x) \ \ \ \ \ (1)

for any fixed {A>0}, where {I = (w,x^{\delta})}, {{\mathcal S}_I} are the square-free integers whose prime factors lie in {I}, and {\Delta_{b,W}(\Lambda;q,a)} is the quantity

\displaystyle  \Delta_{b,W}(\Lambda;q,a) := | \sum_{x \leq n \leq 2x: n=b\ (W); n = a\ (q)} \Lambda(n) \ \ \ \ \ (2)

\displaystyle  - \frac{1}{\phi(q)} \sum_{x \leq n \leq 2x: n = b\ (W)} \Lambda(n)|.

and {C(q)} is the set of congruence classes

\displaystyle  C(q) := \{ a \in ({\bf Z}/q{\bf Z})^\times: P(a) = 0 \}

and {P} is the polynomial

\displaystyle  P(a) := \prod_{h \in {\mathcal H}} (a+h).

The conjecture {MPZ[\varpi,\delta]} is currently known to hold whenever {87 \varpi + 17 \delta < \frac{1}{4}} (see this comment and this confirmation). Actually, we can prove a stronger result than {MPZ[\varpi,\delta]} in this regime in a couple ways. Firstly, the congruence classes {C(q)} can be replaced by a more general system of congruence classes obeying a certain controlled multiplicity axiom; see this post. Secondly, and more importantly for this post, the requirement that the modulus {q} lies in {{\mathcal S}_I} can be relaxed; see below.

To connect the two conjectures, the previously best known implication was the folowing (see Theorem 2 from this post):

Theorem 3 Let {0 < \varpi < 1/4}, {0 < \delta < 1/4 + \varpi} and {k_0 \geq 2} be such that we have the inequality

\displaystyle  (1 +4 \varpi) (1-\kappa') > \frac{j^2_{k_0-2}}{k_0(k_0-1)} (1+\kappa) \ \ \ \ \ (3)

where {j_{k_0-2} = j_{k_0-2,1}} is the first positive zero of the Bessel function {J_{k_0-2}}, and {\kappa,\kappa'>0} are the quantities

\displaystyle  \kappa := \sum_{1 \leq n < \frac{1+4\varpi}{4\delta}} \frac{3^n+1}{2} \frac{k_0^n}{n!} (\int_{4\delta/(1+4\varpi)}^1 (1-t)^{k_0/2}\ \frac{dt}{t})^n

and

\displaystyle  \kappa' := \sum_{2 \leq n < \frac{1+4\varpi}{4\delta}} \frac{3^n-1}{2} \frac{(k_0-1)^n}{n!}

\displaystyle (\int_{4\delta/(1+4\varpi)}^1 (1-t)^{(k_0-1)/2}\ \frac{dt}{t})^n.

Then {MPZ[\varpi,\delta]} implies {DHL[k_0,2]}.

Actually there have been some slight improvements to the quantities {\kappa,\kappa'}; see the comments to this previous post. However, the main error {\kappa} remains roughly of the order {\delta^{-1} \exp( - 2 k_0\delta )}, which limits one from taking {\delta} too small.

To improve beyond this, the first basic observation is that the smoothness condition {q \in {\mathcal S}_I}, which implies that all prime divisors of {q} are less than {x^\delta}, can be relaxed in the proof of {MPZ[\varpi,\delta]}. Indeed, if one inspects the proof of this proposition (described in these three previous posts), one sees that the key property of {q} needed is not so much the smoothness, but a weaker condition which we will call (for lack of a better term) dense divisibility:

Definition 4 Let {y > 1}. A positive integer {q} is said to be {y}-densely divisible if for every {1 \leq R \leq q}, one can find a factor of {q} in the interval {[y^{-1} R, R]}. We let {{\mathcal D}_y} denote the set of positive integers that are {y}-densely divisible.

Certainly every integer which is {y}-smooth (i.e. has all prime factors at most {y} is also {y}-densely divisible, as can be seen from the greedy algorithm; but the property of being {y}-densely divisible is strictly weaker than {y}-smoothness, which is a fact we shall exploit shortly.

We now define {MPZ'[\varpi,\delta]} to be the same statement as {MPZ[\varpi,\delta]}, but with the condition {q \in {\mathcal S}_I} replaced by the weaker condition {q \in {\mathcal S}_{[w,+\infty)} \cap {\mathcal D}_{x^\delta}}. The arguments in previous posts then also establish {MPZ'[\varpi,\delta]} whenever {87 \varpi + 17 \delta < \frac{1}{4}}.

The main result of this post is then the following implication, essentially due to Pintz:

Theorem 5 Let {0 < \varpi < 1/4}, {0 < \delta \leq \delta' < 1/4 + \varpi}, {A \geq 0}, and {k_0 \geq 2} be such that

\displaystyle  (1 +4 \varpi) (1-2\kappa_1 - 2\kappa_2 - 2\kappa_3) > \frac{j^2_{k_0-2}}{k_0(k_0-1)}

where

\displaystyle  \kappa_1 := \int_{\theta}^1 (1-t)^{(k_0-1)/2}\ \frac{dt}{t}

\displaystyle  \kappa_2 := (k_0-1) \int_{\theta}^1 (1-t)^{k_0-1}\ \frac{dt}{t}

\displaystyle  \kappa_3 := e^A \frac{G_{k_0-1,\tilde \theta}(0,0)}{G_{k_0-1}(0,0)} \sum_{0 \leq J \leq 1/\tilde \delta} \frac{(k_0-1)^J}{J!} (\int_{\tilde \delta}^\theta e^{-At} \frac{dt}{t})^J

and

\displaystyle  \theta := \frac{\delta'}{1/4+\varpi}

\displaystyle  \tilde \theta := \frac{(\delta' - \delta)/2 + \varpi}{1/4 + \varpi}

\displaystyle  \tilde \delta := \frac{\delta}{1/4+\varpi}

\displaystyle  G_{k_0-1}(0,0) := \int_0^1 f(t)^2 \frac{t^{k_0-2}}{(k_0-2)!}\ dt

\displaystyle  G_{k_0-1,\tilde \theta}(0,0) := \int_0^{\tilde \theta} f(t)^2 \frac{t^{k_0-2}}{(k_0-2)!}\ dt

and

\displaystyle  f(t) := t^{1-k_0/2} J_{k_0-2}( \sqrt{t} j_{k_0-2} ).

Then {MPZ'[\varpi,\delta]} implies {DHL[k_0,2]}.

This theorem has rather messy constants, but we can isolate some special cases which are a bit easier to compute with. Setting {\delta' = \delta}, we see that {\kappa_3} vanishes (and the argument below will show that we only need {MPZ[\varpi,\delta]} rather than {MPZ'[\varpi,\delta]}), and we obtain the following slight improvement of Theorem 3:

Theorem 6 Let {0 < \varpi < 1/4}, {0 < \delta < 1/4 + \varpi} and {k_0 \geq 2} be such that we have the inequality

\displaystyle  (1 +4 \varpi) (1-2\kappa_1-2\kappa_2) > \frac{j^2_{k_0-2}}{k_0(k_0-1)} \ \ \ \ \ (4)

where

\displaystyle  \kappa_1 := \int_{4\delta/(1+4\varpi)}^1 (1-t)^{(k_0-1)/2}\ \frac{dt}{t}

\displaystyle  \kappa_2 := (k_0-1) \int_{4\delta/(1+4\varpi)}^1 (1-t)^{k_0-1}\ \frac{dt}{t}.

Then {MPZ[\varpi,\delta]} implies {DHL[k_0,2]}.

This is a little better than Theorem 3, because the error {2\kappa_1+2\kappa_2} has size about {\frac{1}{2 k_0 \delta} \exp( - 2 k_0 \delta) + \frac{1}{2 \delta} \exp(-4 k_0 \delta)}, which compares favorably with the error in Theorem 3 which is about {\frac{1}{\delta} \exp(-2 k_0 \delta)}. This should already give a “cheap” improvement to our current threshold {k_0 \geq 6329}, though it will fall short of what one would get if one fully optimised over all parameters in the above theorem.

Returning to the full strength of Theorem 5, let us obtain a crude upper bound for {\kappa_3} that is a little simpler to understand. Extending the {J} summation to infinity and using the Taylor series for the exponential, we have

\displaystyle  \kappa_3 \leq \frac{G_{k_0-1,\tilde \theta}(0,0)}{G_{k_0-1}(0,0)} \exp( A + (k_0-1) \int_{\tilde \delta}^\theta e^{-At} \frac{dt}{t} ).

We can crudely bound

\displaystyle  \int_{\tilde \delta}^\theta e^{-At} \frac{dt}{t} \leq \frac{1}{A \tilde \delta}

and then optimise in {A} to obtain

\displaystyle  \kappa_3 \leq \frac{G_{k_0-1,\tilde \theta}(0,0)}{G_{k_0-1}(0,0)} \exp( 2 (k_0-1)^{1/2} \tilde \delta^{-1/2} ).

Because of the {t^{k_0-2}} factor in the integrand for {G_{k_0-1}} and {G_{k_0-1,\tilde \theta}}, we expect the ratio {\frac{G_{k_0-1,\tilde \theta}(0,0)}{G_{k_0-1}(0,0)}} to be of the order of {\tilde \theta^{k_0-1}}, although one will need some theoretical or numerical estimates on Bessel functions to make this heuristic more precise. Setting {\tilde \theta} to be something like {1/2}, we get a good bound here as long as {\tilde \delta \gg 1/k_0}, which at current values of {\delta, k_0} is a mild condition.

Pintz’s argument uses the elementary Selberg sieve, discussed in this previous post, but with a more efficient estimation of the quantity {\beta}, in particular avoiding the truncation to moduli {d} between {x^{-\delta} R} and {R} which was the main source of inefficiency in that previous post. The basic idea is to “linearise” the effect of the truncation of the sieve, so that this contribution can be dealt with by the union bound (basically, bounding the contribution of each large prime one at a time). This mostly avoids the more complicated combinatorial analysis that arose in the analytic Selberg sieve, as seen in this previous post.

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This post is a continuation of the previous post on sieve theory, which is an ongoing part of the Polymath8 project to improve the various parameters in Zhang’s proof that bounded gaps between primes occur infinitely often. Given that the comments on that page are getting quite lengthy, this is also a good opportunity to “roll over” that thread.

We will continue the notation from the previous post, including the concept of an admissible tuple, the use of an asymptotic parameter {x} going to infinity, and a quantity {w} depending on {x} that goes to infinity sufficiently slowly with {x}, and {W := \prod_{p<w} p} (the {W}-trick).

The objective of this portion of the Polymath8 project is to make as efficient as possible the connection between two types of results, which we call {DHL[k_0,2]} and {MPZ[\varpi,\delta]}. Let us first state {DHL[k_0,2]}, which has an integer parameter {k_0 \geq 2}:

Conjecture 1 ({DHL[k_0,2]}) Let {{\mathcal H}} be a fixed admissible {k_0}-tuple. Then there are infinitely many translates {n+{\mathcal H}} of {{\mathcal H}} which contain at least two primes.

Zhang was the first to prove a result of this type with {k_0 = 3,500,000}. Since then the value of {k_0} has been lowered substantially; at this time of writing, the current record is {k_0 = 26,024}.

There are two basic ways known currently to attain this conjecture. The first is to use the Elliott-Halberstam conjecture {EH[\theta]} for some {\theta>1/2}:

Conjecture 2 ({EH[\theta]}) One has

\displaystyle  \sum_{1 \leq q \leq x^\theta} \sup_{a \in ({\bf Z}/q{\bf Z})^\times} |\sum_{n < x: n = a\ (q)} \Lambda(n) - \frac{1}{\phi(q)} \sum_{n < x} \Lambda(n)|

\displaystyle = O( \frac{x}{\log^A x} )

for all fixed {A>0}. Here we use the abbreviation {n=a\ (q)} for {n=a \hbox{ mod } q}.

Here of course {\Lambda} is the von Mangoldt function and {\phi} the Euler totient function. It is conjectured that {EH[\theta]} holds for all {0 < \theta < 1}, but this is currently only known for {0 < \theta < 1/2}, an important result known as the Bombieri-Vinogradov theorem.

In a breakthrough paper, Goldston, Yildirim, and Pintz established an implication of the form

\displaystyle  EH[\theta] \implies DHL[k_0,2] \ \ \ \ \ (1)

for any {1/2 < \theta < 1}, where {k_0 = k_0(\theta)} depends on {\theta}. This deduction was very recently optimised by Farkas, Pintz, and Revesz and also independently in the comments to the previous blog post, leading to the following implication:

Theorem 3 (EH implies DHL) Let {1/2 < \theta < 1} be a real number, and let {k_0 \geq 2} be an integer obeying the inequality

\displaystyle  2\theta > \frac{j_{k_0-2}^2}{k_0(k_0-1)}, \ \ \ \ \ (2)

where {j_n} is the first positive zero of the Bessel function {J_n(x)}. Then {EH[\theta]} implies {DHL[k_0,2]}.

Note that the right-hand side of (2) is larger than {1}, but tends asymptotically to {1} as {k_0 \rightarrow \infty}. We give an alternate proof of Theorem 3 below the fold.

Implications of the form Theorem 3 were modified by Motohashi and Pintz, which in our notation replaces {EH[\theta]} by an easier conjecture {MPZ[\varpi,\delta]} for some {0 < \varpi < 1/4} and {0 < \delta < 1/4+\varpi}, at the cost of degrading the sufficient condition (2) slightly. In our notation, this conjecture takes the following form for each choice of parameters {\varpi,\delta}:

Conjecture 4 ({MPZ[\varpi,\delta]}) Let {{\mathcal H}} be a fixed {k_0}-tuple (not necessarily admissible) for some fixed {k_0 \geq 2}, and let {b\ (W)} be a primitive residue class. Then

\displaystyle  \sum_{q \in {\mathcal S}_I: q< x^{1/2+2\varpi}} \sum_{a \in C(q)} |\Delta_{b,W}(\Lambda; q,a)| = O( x \log^{-A} x) \ \ \ \ \ (3)

for any fixed {A>0}, where {I = (w,x^{\delta})}, {{\mathcal S}_I} are the square-free integers whose prime factors lie in {I}, and {\Delta_{b,W}(\Lambda;q,a)} is the quantity

\displaystyle  \Delta_{b,W}(\Lambda;q,a) := | \sum_{x \leq n \leq 2x: n=b\ (W); n = a\ (q)} \Lambda(n) \ \ \ \ \ (4)

\displaystyle  - \frac{1}{\phi(q)} \sum_{x \leq n \leq 2x: n = b\ (W)} \Lambda(n)|.

and {C(q)} is the set of congruence classes

\displaystyle  C(q) := \{ a \in ({\bf Z}/q{\bf Z})^\times: P(a) = 0 \}

and {P} is the polynomial

\displaystyle  P(a) := \prod_{h \in {\mathcal H}} (a+h).

This is a weakened version of the Elliott-Halberstam conjecture:

Proposition 5 (EH implies MPZ) Let {0 < \varpi < 1/4} and {0 < \delta < 1/4+\varpi}. Then {EH[1/2+2\varpi+\epsilon]} implies {MPZ[\varpi,\delta]} for any {\epsilon>0}. (In abbreviated form: {EH[1/2+2\varpi+]} implies {MPZ[\varpi,\delta]}.)

In particular, since {EH[\theta]} is conjecturally true for all {0 < \theta < 1/2}, we conjecture {MPZ[\varpi,\delta]} to be true for all {0 < \varpi < 1/4} and {0<\delta<1/4+\varpi}.

Proof: Define

\displaystyle  E(q) := \sup_{a \in ({\bf Z}/q{\bf Z})^\times} |\sum_{x \leq n \leq 2x: n = a\ (q)} \Lambda(n) - \frac{1}{\phi(q)} \sum_{x \leq n \leq 2x} \Lambda(n)|

then the hypothesis {EH[1/2+2\varpi+\epsilon]} (applied to {x} and {2x} and then subtracting) tells us that

\displaystyle  \sum_{1 \leq q \leq Wx^{1/2+2\varpi}} E(q) \ll x \log^{-A} x

for any fixed {A>0}. From the Chinese remainder theorem and the Siegel-Walfisz theorem we have

\displaystyle  \sup_{a \in ({\bf Z}/q{\bf Z})^\times} \Delta_{b,W}(\Lambda;q,a) \ll E(qW) + \frac{1}{\phi(q)} x \log^{-A} x

for any {q} coprime to {W} (and in particular for {q \in {\mathcal S}_I}). Since {|C(q)| \leq k_0^{\Omega(q)}}, where {\Omega(q)} is the number of prime divisors of {q}, we can thus bound the left-hand side of (3) by

\displaystyle  \ll \sum_{q \in {\mathcal S}_I: q< x^{1/2+2\varpi}} k_0^{\Omega(q)} E(qW) + k_0^{\Omega(q)} \frac{1}{\phi(q)} x \log^{-A} x.

The contribution of the second term is {O(x \log^{-A+O(1)} x)} by standard estimates (see Proposition 8 below). Using the very crude bound

\displaystyle  E(q) \ll \frac{1}{\phi(q)} x \log x

and standard estimates we also have

\displaystyle  \sum_{q \in {\mathcal S}_I: q< x^{1/2+2\varpi}} k_0^{2\Omega(q)} E(qW) \ll x \log^{O(1)} A

and the claim now follows from the Cauchy-Schwarz inequality. \Box

In practice, the conjecture {MPZ[\varpi,\delta]} is easier to prove than {EH[1/2+2\varpi+]} due to the restriction of the residue classes {a} to {C(q)}, and also the restriction of the modulus {q} to {x^\delta}-smooth numbers. Zhang proved {MPZ[\varpi,\varpi]} for any {0 < \varpi < 1/1168}. More recently, our Polymath8 group has analysed Zhang’s argument (using in part a corrected version of the analysis of a recent preprint of Pintz) to obtain {MPZ[\varpi,\delta]} whenever {\delta, \varpi > 0} are such that

\displaystyle  207\varpi + 43\delta < \frac{1}{4}.

The work of Motohashi and Pintz, and later Zhang, implicitly describe arguments that allow one to deduce {DHL[k_0,2]} from {MPZ[\varpi,\delta]} provided that {k_0} is sufficiently large depending on {\varpi,\delta}. The best implication of this sort that we have been able to verify thus far is the following result, established in the previous post:

Theorem 6 (MPZ implies DHL) Let {0 < \varpi < 1/4}, {0 < \delta < 1/4+\varpi}, and let {k_0 \geq 2} be an integer obeying the constraint

\displaystyle  1+4\varpi > \frac{j_{k_0-2}^2}{k_0(k_0-1)} (1+\kappa) \ \ \ \ \ (5)

where {\kappa} is the quantity

\displaystyle \kappa := \sum_{1 \leq n < \frac{1+4\varpi}{2\delta}} (1 - \frac{2n \delta}{1 + 4\varpi})^{k_0/2} \prod_{j=1}^{n} (1 + 3k_0 \log(1+\frac{1}{j})) ).

Then {MPZ[\varpi,\delta]} implies {DHL[k_0,2]}.

This complicated version of {\kappa} is roughly of size {3 \log(2) k_0 \exp( - k_0 \delta)}. It is unlikely to be optimal; the work of Motohashi-Pintz and Pintz suggests that it can essentially be improved to {\frac{1}{\delta} \exp(-k_0 \delta)}, but currently we are unable to verify this claim. One of the aims of this post is to encourage further discussion as to how to improve the {\kappa} term in results such as Theorem 6.

We remark that as (5) is an open condition, it is unaffected by infinitesimal modifications to {\varpi,\delta}, and so we do not ascribe much importance to such modifications (e.g. replacing {\varpi} by {\varpi-\epsilon} for some arbitrarily small {\epsilon>0}).

The known deductions of {DHL[k_0,2]} from claims such as {EH[\theta]} or {MPZ[\varpi,\delta]} rely on the following elementary observation of Goldston, Pintz, and Yildirim (essentially a weighted pigeonhole principle), which we have placed in “{W}-tricked form”:

Lemma 7 (Criterion for DHL) Let {k_0 \geq 2}. Suppose that for each fixed admissible {k_0}-tuple {{\mathcal H}} and each congruence class {b\ (W)} such that {b+h} is coprime to {W} for all {h \in {\mathcal H}}, one can find a non-negative weight function {\nu: {\bf N} \rightarrow {\bf R}^+}, fixed quantities {\alpha,\beta > 0}, a quantity {A>0}, and a fixed positive power {R} of {x} such that one has the upper bound

\displaystyle  \sum_{x \leq n \leq 2x: n = b\ (W)} \nu(n) \leq (\alpha+o(1)) A\frac{x}{W}, \ \ \ \ \ (6)

the lower bound

\displaystyle  \sum_{x \leq n \leq 2x: n = b\ (W)} \nu(n) \theta(n+h_i) \geq (\beta-o(1)) A\frac{x}{W} \log R \ \ \ \ \ (7)

for all {h_i \in {\mathcal H}}, and the key inequality

\displaystyle  \frac{\log R}{\log x} > \frac{1}{k_0} \frac{\alpha}{\beta} \ \ \ \ \ (8)

holds. Then {DHL[k_0,2]} holds. Here {\theta(n)} is defined to equal {\log n} when {n} is prime and {0} otherwise.

Proof: Consider the quantity

\displaystyle  \sum_{x \leq n \leq 2x: n = b\ (W)} \nu(n) (\sum_{h \in {\mathcal H}} \theta(n+h) - \log(3x)). \ \ \ \ \ (9)

By (6), (7), this quantity is at least

\displaystyle  k_0 \beta A\frac{x}{W} \log R - \alpha \log(3x) A\frac{x}{W} - o(A\frac{x}{W} \log x).

By (8), this expression is positive for all sufficiently large {x}. On the other hand, (9) can only be positive if at least one summand is positive, which only can happen when {n+{\mathcal H}} contains at least two primes for some {x \leq n \leq 2x} with {n=b\ (W)}. Letting {x \rightarrow \infty} we obtain {DHL[k_0,2]} as claimed. \Box

In practice, the quantity {R} (referred to as the sieve level) is a power of {x} such as {x^{\theta/2}} or {x^{1/4+\varpi}}, and reflects the strength of the distribution hypothesis {EH[\theta]} or {MPZ[\varpi,\delta]} that is available; the quantity {R} will also be a key parameter in the definition of the sieve weight {\nu}. The factor {A} reflects the order of magnitude of the expected density of {\nu} in the residue class {b\ (W)}; it could be absorbed into the sieve weight {\nu} by dividing that weight by {A}, but it is convenient to not enforce such a normalisation so as not to clutter up the formulae. In practice, {A} will some combination of {\frac{\phi(W)}{W}} and {\log R}.

Once one has decided to rely on Lemma 7, the next main task is to select a good weight {\nu} for which the ratio {\alpha/\beta} is as small as possible (and for which the sieve level {R} is as large as possible. To ensure non-negativity, we use the Selberg sieve

\displaystyle  \nu = \lambda^2, \ \ \ \ \ (10)

where {\lambda(n)} takes the form

\displaystyle  \lambda(n) = \sum_{d \in {\mathcal S}_I: d|P(n)} \mu(d) a_d

for some weights {a_d \in {\bf R}} vanishing for {d>R} that are to be chosen, where {I \subset (w,+\infty)} is an interval and {P} is the polynomial {P(n) := \prod_{h \in {\mathcal H}} (n+h)}. If the distribution hypothesis is {EH[\theta]}, one takes {R := x^{\theta/2}} and {I := (w,+\infty)}; if the distribution hypothesis is instead {MPZ[\varpi,\delta]}, one takes {R := x^{1/4+\varpi}} and {I := (w,x^\delta)}.

One has a useful amount of flexibility in selecting the weights {a_d} for the Selberg sieve. The original work of Goldston, Pintz, and Yildirim, as well as the subsequent paper of Zhang, the choice

\displaystyle  a_d := \log(\frac{R}{d})_+^{k_0+\ell_0}

is used for some additional parameter {\ell_0 > 0} to be optimised over. More generally, one can take

\displaystyle  a_d := g( \frac{\log d}{\log R} )

for some suitable (in particular, sufficiently smooth) cutoff function {g: {\bf R} \rightarrow {\bf R}}. We will refer to this choice of sieve weights as the “analytic Selberg sieve”; this is the choice used in the analysis in the previous post.

However, there is a slight variant choice of sieve weights that one can use, which I will call the “elementary Selberg sieve”, and it takes the form

\displaystyle  a_d := \frac{1}{\Phi(d) \Delta(d)} \sum_{q \in {\mathcal S}_I: (q,d)=1} \frac{1}{\Phi(q)} f'( \frac{\log dq}{\log R}) \ \ \ \ \ (11)

for a sufficiently smooth function {f: {\bf R} \rightarrow {\bf R}}, where

\displaystyle  \Phi(d) := \prod_{p|d} \frac{p-k_0}{k_0}

for {d \in {\mathcal S}_I} is a {k_0}-variant of the Euler totient function, and

\displaystyle  \Delta(d) := \prod_{p|d} \frac{k_0}{p} = \frac{k_0^{\Omega(d)}}{d}

for {d \in {\mathcal S}_I} is a {k_0}-variant of the function {1/d}. (The derivative on the {f} cutoff is convenient for computations, as will be made clearer later in this post.) This choice of weights {a_d} may seem somewhat arbitrary, but it arises naturally when considering how to optimise the quadratic form

\displaystyle  \sum_{d_1,d_2 \in {\mathcal S}_I} \mu(d_1) a_{d_1} \mu(d_2) a_{d_2} \Delta([d_1,d_2])

(which arises naturally in the estimation of {\alpha} in (6)) subject to a fixed value of {a_1} (which morally is associated to the estimation of {\beta} in (7)); this is discussed in any sieve theory text as part of the general theory of the Selberg sieve, e.g. Friedlander-Iwaniec.

The use of the elementary Selberg sieve for the bounded prime gaps problem was studied by Motohashi and Pintz. Their arguments give an alternate derivation of {DHL[k_0,2]} from {MPZ[\varpi,\theta]} for {k_0} sufficiently large, although unfortunately we were not able to confirm some of their calculations regarding the precise dependence of {k_0} on {\varpi,\theta}, and in particular we have not yet been able to improve upon the specific criterion in Theorem 6 using the elementary sieve. However it is quite plausible that such improvements could become available with additional arguments.

Below the fold we describe how the elementary Selberg sieve can be used to reprove Theorem 3, and discuss how they could potentially be used to improve upon Theorem 6. (But the elementary Selberg sieve and the analytic Selberg sieve are in any event closely related; see the appendix of this paper of mine with Ben Green for some further discussion.) For the purposes of polymath8, either developing the elementary Selberg sieve or continuing the analysis of the analytic Selberg sieve from the previous post would be a relevant topic of conversation in the comments to this post.

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In this, the final lecture notes of this course, we discuss one of the motivating applications of the theory developed thus far, namely to count solutions to linear equations in primes {{\mathcal P} = \{2,3,5,7,\ldots\}} (or in dense subsets {A} of primes {{\mathcal P}}). Unfortunately, the most famous linear equations in primes: the twin prime equation {p_2 - p_1 = 2} and the even Goldbach equation {p_1+p_2=N} – remain out of reach of this technology (because the relevant affine linear forms involved are commensurate, and thus have infinite complexity with respect to the Gowers norms), but most other systems of equations, in particular that of arithmetic progressions {p_i = n+ir} for {i=0,\ldots,k-1} (or equivalently, {p_i + p_{i+2} = 2p_{i+1}} for {i=0,\ldots,k-2}) , as well as the odd Goldbach equation {p_1+p_2+p_3=N}, are tractable.

To illustrate the main ideas, we will focus on the following result of Green:

Theorem 1 (Roth’s theorem in the primes) Let {A \subset {\mathcal P}} be a subset of primes whose upper density {\limsup_{N \rightarrow \infty} |A \cap [N]|/|{\mathcal P} \cap [N]|} is positive. Then {A} contains infinitely many arithmetic progressions of length three.

This should be compared with Roth’s theorem in the integers (Notes 2), which is the same statement but with the primes {{\mathcal P}} replaced by the integers {{\bf Z}} (or natural numbers {{\bf N}}). Indeed, Roth’s theorem for the primes is proven by transferring Roth’s theorem for the integers to the prime setting; the latter theorem is used as a “black box”. The key difficulty here in performing this transference is that the primes have zero density inside the integers; indeed, from the prime number theorem we have {|{\mathcal P} \cap [N]| = (1+o(1)) \frac{N}{\log N} = o(N)}.

There are a number of generalisations of this transference technique. In a paper of Green and myself, we extended the above theorem to progressions of longer length (thus transferring Szemerédi’s theorem to the primes). In a series of papers (culminating in a paper to appear shortly) of Green, myself, and also Ziegler, related methods are also used to obtain an asymptotic for the number of solutions in the primes to any system of linear equations of bounded complexity. This latter result uses the full power of higher order Fourier analysis, in particular relying heavily on the inverse conjecture for the Gowers norms; in contrast, Roth’s theorem and Szemerédi’s theorem in the primes are “softer” results that do not need this conjecture.

To transfer results from the integers to the primes, there are three basic steps:

  • A general transference principle, that transfers certain types of additive combinatorial results from dense subsets of the integers to dense subsets of a suitably “pseudorandom set” of integers (or more precisely, to the integers weighted by a suitably “pseudorandom measure”);
  • An application of sieve theory to show that the primes (or more precisely, an affine modification of the primes) lie inside a suitably pseudorandom set of integers (or more precisely, have significant mass with respect to a suitably pseudorandom measure).
  • If one is seeking asymptotics for patterns in the primes, and not simply lower bounds, one also needs to control correlations between the primes (or proxies for the primes, such as the Möbius function) with various objects that arise from higher order Fourier analysis, such as nilsequences.

The former step can be accomplished in a number of ways. For progressions of length three (and more generally, for controlling linear patterns of complexity at most one), transference can be accomplished by Fourier-analytic methods. For more complicated patterns, one can use techniques inspired by ergodic theory; more recently, simplified and more efficient methods based on duality (the Hahn-Banach theorem) have also been used. No number theory is used in this step. (In the case of transference to genuinely random sets, rather than pseudorandom sets, similar ideas appeared earlier in the graph theory setting, see this paper of Kohayakawa, Luczak, and Rodl.

The second step is accomplished by fairly standard sieve theory methods (e.g. the Selberg sieve, or the slight variants of this sieve used by Goldston and Yildirim). Remarkably, very little of the formidable apparatus of modern analytic number theory is needed for this step; for instance, the only fact about the Riemann zeta function that is truly needed is that it has a simple pole at {s=1}, and no knowledge of L-functions is needed.

The third step does draw more significantly on analytic number theory techniques and results (most notably, the method of Vinogradov to compute oscillatory sums over the primes, and also the Siegel-Walfisz theorem that gives a good error term on the prime number theorem in arithemtic progressions). As these techniques are somewhat orthogonal to the main topic of this course, we shall only touch briefly on this aspect of the transference strategy.

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I’ve just uploaded to the arXiv the short note “A remark on primality testing and the binary expansion“, submitted to the Journal of the Australian Mathematical Society. In this note I establish the following result: for any sufficiently large integer n, there exists an n-bit prime p, such that the numbers p \pm 2^i for i=0,\ldots,n-1 are all composite. In particular, if one flips any one of the bits in the binary expansion of the prime p, one obtains a composite number. As a consequence, one obtains the (rather plausible) consequence that in order to (deterministically) test whether an n-bit integer is prime or not, one needs (in the worst-case) to read all n bits of the prime. (This question was posed to me by my colleague here at UCLA, Yiannis Moschovakis.)

Primes p of the form mentioned in the above result are somewhat rare at first; the first some prime is 1973 = 11110110101_2. But in fact, the argument in my note shows that the set of such primes actually has positive relative density inside the set of all primes. (Amusingly, this means that one can apply a theorem of Ben Green and myself and conclude that there are arbitrarily long arithmetic progressions of such primes, although I doubt that there is any particular significance or application to this conclusion.)

The same remark applies to other bases; thus, for instance, there exist infinitely many prime numbers with the property that if one changes any one of the base 10 digits of that number, one obtains a composite number. (Presumably the first such number can be located by computer search, though I did not attempt to do so.) [Update, Feb 25: see comments.]

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