Sharp Tilting Inequalities

For positive semidefinite matrices $A,B$ and $1/2\leα\le1$, we establish $\operatorname{Tr}\!\left[A\,D\log(A^α+B^α)(B^α)\right] \le s^s(1-s)^{1-s}\operatorname{Tr}[A^αB^{1-α}]$, where $s=(1-α)/α$, with the optimal coefficient. The proof uses a superoperator representation of the directional derivative of the logarithm and an Araki-type estimate for the derivative of a fractional power. Both this inequality and its symmetric-quotient counterpart extend to positive trace-class $A$ and positive, possibly unbounded, self-adjoint $B$ on a separable Hilbert space.