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 $f$.
Categories: math.FA