Choose a unit vector $e$ and define the rank-one operator
$$
Th=-\langle h,e\rangle e.
$$
It is bounded, self-adjoint, and <Hilbert-Schmidt operator>[Hilbert-Schmidt], with $\lVert T\rVert_{\rm HS}=1$. But
$$
\langle Te,e\rangle=-1<0,
$$
so it is not positive. Since every covariance operator is positive, $T$ is the required counterexample.

Solution

Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 225 / 1 / c / Solution

 Solution

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