Richard Oberlin, Andreas Seeger, Christoph Thiele, Jim Wright, and I have just uploaded to the arXiv our paper “A variation norm Carleson theorem“, submitted to J. Europ. Math. Soc..

The celebrated Carleson-Hunt theorem asserts that if f: {\Bbb R}/{\Bbb Z} \to {\Bbb C} is an L^p function for some 1 < p \leq \infty, then the partial Fourier series

S_n f(x) := \sum_{k=-n}^n \hat f(k) e^{2\pi ikx}

of f converge to f almost everywhere.  (The claim fails for p=1, as shown by a famous counterexample of Kolmogorov.)  The theorem follows easily from the inequality

\| \sup_n |S_n f(x)| \|_{L^p({\Bbb R}/{\Bbb Z})} \leq C_p \| f \|_{L^p({\Bbb R}/{\Bbb Z})}, (1)

where 1 < p < \infty, and C_p depends only on p.  Indeed, one first verifies Carleson’s theorem for a dense subclass of L^p (e.g. the space of test functions) and then uses a standard limiting argument involving (1) (this is an example of the trick “give yourself an epsilon of room“).

The Carleson-Hunt theorem shows that S_n f(x) converges as n \to \infty for almost every x, but does not say much more about the nature of that convergence.  One way to measure the strength of the convergence is to introduce the variational norms

{\mathcal V}^r (S_n f(x))_{n = 0}^\infty := \sup_{n_1 \leq \ldots \leq n_k} (\sum_{j=1}^{k-1} |S_{n_{j+1}} f(x) - S_{n_j} f(x)|^r)^{1/r}

for various 1 \leq r \leq \infty.  For r = \infty this is the Carleson maximal function \sup_n |S_n f(x)|; for r=1 this is the total variation of the sequence S_n f(x), which one can verify to be the \ell^1 norm of \hat f.

Our main result is to obtain the following variational strengthening of (1)

\| {\mathcal V}^r S_n f(x) \|_{L^p({\Bbb R}/{\Bbb Z})} \leq C_{p,r} \| f \|_{L^p({\Bbb R}/{\Bbb Z})} (2)

whenever r > 2 and r/(r-1) < p < \infty; these conditions on p,r are optimal. (For those readers familiar with martingales, the relationship of (2) to (1) is analogous to the relationship between Lepingle’s inequality (a variant of the more well known Doob’s inequality) and the Hardy-Littlewood maximal inequality.)

Because a sequence with finite r-variation for some finite r is necessarily convergent, this leads to a new proof of the Carleson-Hunt theorem without the need for a dense subclass.  In particular, we obtain ergodic theory analogues of this result, in the case where no obvious dense subclass is available; more precisely, we obtain a new (and more “quantitative”) proof of a Wiener-Wintner-type theorem (first obtained by Lacey and Terwilleger), namely that given any measure-preserving group (T_t)_{t \in {\Bbb R}} on a measure space X, and a function f \in L^p(X) for some p>1, one has for almost every x \in X that for every real number \theta, the integrals \int_{\varepsilon \leq |t| \leq N} T^t f(x) e^{i\theta t}/t\ dt converge as \varepsilon \to 0, N \to \infty for every \theta (not merely almost every \theta).

The estimate (2) also provides a new proof of a result of Christ and Kiselev on the almost everywhere boundedness of eigenfunctions of Schrodinger operators with L^p potentials with p<2.  Unfortunately, due to various endpoint issues, this barely fails to settle the endpoint case p=2, a conjecture known as the nonlinear Carleson conjecture (discussed in this previous post).

The approach here follows the Lacey-Thiele approach to Carleson’s theorem (which is in turn based on an earlier approach of Fefferman), based on linearising the Carleson maximal function by picking the integer n = n(x) which attains the supremum \sup_n |S_n f(x)|, dividing phase space into “tiles”, and organising these tiles into “trees” and then into “forests” based on the distribution of the phase space “energy” of f, together with the “mass” distribution of the graph of the function x \mapsto n(x).  One then needs to combine various “Bessel” type bounds on the energy, “Vitali-type” bounds on the mass, and “Calderon-Zygmund” type estimates on the trees together to obtain the result.

In our setting, the main new difficulty is that there are multiple integers n_1(x),\ldots,n_{k(x)}(x) associated to each point rather than one, which requires a more detailed analysis of the “multiplicity” of forests that was not present in earlier work.  (Also, the Calderon-Zygmund estimates need to be replaced with Lepingle type estimates, though this is a relatively standard change, being first introduced in a paper of Bourgain.)