On the fourth order Schrodinger equation in four dimensions: Dispersive estimates and zero energy resonances

We study the fourth order Schrodinger operator H = (-Delta)(2) + V for a decaying potential V in four dimensions. In particular, we show that the t(-1) decay rate holds in the L-1 -> L-infinity setting if zero energy is regular. Furthermore, if the threshold energies are regular then a faster decay rate of t(-1) (log t)(-2) is attained for large t, at the cost of logarithmic spatial weights. Zero is not regular for the free equation, hence the free evolution does not satisfy this bound due to the presence of a resonance at the zero energy. We provide a full classification of the different types of zero energy resonances and study the effect of each type on the time decay in the dispersive bounds. (C) 2019 Elsevier Inc. All rights reserved.