# Perturbation theory and higher order $\mathcal{S}^p$-differentiability of operator functions

## TL;DR

This paper develops higher order differentiability results for operator functions in Schatten classes, providing explicit formulas and estimates, extending previous work to broader classes of functions and Schatten classes.

## Contribution

It extends higher order differentiability results of operator functions to $ ext{S}^p$ classes and less smooth functions, with explicit derivative formulas and remainders.

## Key findings

- Established $n$-th order $ ext{S}^p$-differentiability for operator functions
- Provided explicit formulas for derivatives using multiple operator integrals
- Extended previous results to broader Schatten classes and less smooth functions

## Abstract

We establish, for $1 < p < \infty$, higher order $\mathcal{S}^p$-differentiability results of the function $\varphi : t\in \mathbb{R} \mapsto f(A+tK) - f(A)$ for selfadjoint operators $A$ and $K$ on a separable Hilbert space $\mathcal{H}$ with $K$ element of the Schatten class $\mathcal{S}^p(\mathcal{H})$ and $f$ $n$-times differentiable on $\mathbb{R}$. We prove that if either $A$ and $f^{(n)}$ are bounded or $f^{(i)}, 1 \leq i \leq n$ are bounded, $\varphi$ is $n$-times differentiable on $\mathbb{R}$ in the $\mathcal{S}^p$-norm with bounded $n$th derivative. If $f\in C^n(\mathbb{R})$ with bounded $f^{(n)}$, we prove that $\varphi$ is $n$-times continuously differentiable on $\mathbb{R}$. We give explicit formulas for the derivatives of $\varphi$, in terms of multiple operator integrals. As for application, we establish a formula and $\mathcal{S}^p$-estimates for operator Taylor remainders for a more extensive class of functions. These results are the $n$th order analogue of the results of \cite{KPSS}. They also extend the results of \cite{CLSS} from $\mathcal{S}^2(\mathcal{H})$ to $\mathcal{S}^p(\mathcal{H})$ and the results of \cite{LMS} from $n$-times continuously differentiable functions to $n$-times differentiable functions $f$.