Improved Critical Eigenfunction Restriction Estimates on Riemannian Surfaces with Nonpositive Curvature

We show that one can obtain improved L-4 geodesic restriction estimates for eigenfunctions on compact Riemannian surfaces with nonpositive curvature. We achieve this by adapting Sogge's strategy in (Improved critical eigenfunction estimates on manifolds of nonpositive curvature, Preprint). We first combine the improved L-2 restriction estimate of Blair and Sogge (Concerning Toponogov's Theorem and logarithmic improvement of estimates of eigenfunctions, Preprint) and the classical improved L-infinity estimate of Berard to obtain an improvedweak-type L-4 restriction estimate. We then upgrade this weak estimate to a strong one by using the improved Lorentz space estimate of Bak and Seeger (Math Res Lett 18(4): 767-781, 2011). This estimate improves the L-4 restriction estimate of Burq et al. (Duke Math J 138: 445-486, 2007) and Hu (Forum Math 6: 1021-1052, 2009) by a power of (log log lambda)(-1). Moreover, in the case of compact hyperbolic surfaces, we obtain further improvements in terms of (log lambda)(-1) by applying the ideas from (Chen and Sogge, Commun Math Phys 329(3): 435-459, 2014) and (Blair and Sogge, Concerning Toponogov's Theorem and logarithmic improvement of estimates of eigenfunctions, Preprint). We are able to compute various constants that appeared in (Chen and Sogge, Commun Math Phys 329(3): 435-459, 2014) explicitly, by proving detailed oscillatory integral estimates and lifting calculations to the universal cover H-2.