Derivative Embeddings in Vector-Valued Morrey Spaces

Abstract

We develop an intermediate-derivative theory for Banach-valued evolution classes with Morrey control in time. Let $E_0\hookrightarrow E$ be Banach spaces, $m\ge2$, $1<p<\infty$, and $0\le\lambda<1$. If $u\in\mathcal M^{p,\lambda}(E_0)$ and $u^{(m)}\in\mathcal M^{p,\lambda}(E)$, then every intermediate derivative satisfies \[ u^{(j)}\in\mathcal M^{p,\lambda}\bigl((E_0,E)_{j/m,\infty}\bigr),\qquad 0<j<m, \] with no UMD, Fourier type, or operator-valued multiplier assumption. The proof is based on an exact rational-frequency splitting of $D^j$, the Peetre $K$-functional, and the Hardy--Littlewood maximal operator on Morrey spaces. On the line the estimate has the scale-sharp multiplicative form. On finite intervals we construct a graph-Morrey extension and obtain the same embedding. The weak fine index is optimal for arbitrary Banach couples. Combining the derivative theorem with the exact Morrey trace theorem yields an exact onto trace formula for every $u^{(j)}$, together with mixed H\"older/interpolation embeddings and a trace-powered $p$-to-$q$ Morrey gain. If $E_0\hookc E$, these embeddings become compact both in the same Morrey norm after an arbitrarily small loss of interpolation smoothness and in time-H\"older spaces above the derivative-trace threshold. For positive selfadjoint Hilbert scales we further improve the weak interpolation target to the fractional domain $D(B^{1-j/m})$ and show that this exponent is sharp.