  A very particular by-product of the result announced in the title reads as
follows: Let $(X,<\cdot,\cdot>)$ be a real Hilbert space, $T:X\to X$ a compact
and symmetric linear operator, and $z\in X$ such that the equation
$T(x)-\|T\|x=z$ has no solution in $X$. For each $r>0$, set
$\gamma(r)=\sup_{x\in S_r}J(x)$, where $J(x)=< T(x)-2z,x>$ and $S_r=\{x\in
X:\|x\|^2=r\}$. Then, the function $\gamma$ is $C^1$, increasing and strictly
concave in $]0,+\infty[$, with $\gamma'(]0,+\infty[)=]\|T\|,+\infty[$;
moreover, for each $r>0$, the problem of maximizing $J$ over $S_r$ is
well-posed, and one has $$T(\hat x_r)-\gamma'(r)\hat x_r=z$$ where $\hat x_r$
is the only global maximum of $J_{|S_r}$.\par
