Pre-threshold fractional susceptibility function: holomorphy and response formula

Abstract

For certain smooth unimodal families with negative Schwarzian derivative, we construct a set of Collet-Eckmann and subexponentially recurrent parameters $\Omega$, whose complement set has sufficiently fast decaying density, on which exponential mixing with uniform rates occurs. We use this construction to establish holomorphy of the true fractional susceptibility function of the logistic family, in a disk of radius larger than one, for differentiation index $0\le\eta<1/2$, as recently conjectured by Baladi and Smania. We also obtain a fractional response formula.