EN
We prove the following result: Let X be a real Hilbert space and let J: X → ℝ be a C¹ functional with a nonexpansive derivative. Then, for each r > 0, the following alternative holds: either J' has a fixed point with norm less than r, or
$sup_{||x||=r}J(x) = sup_{||u||_{L²([0,1],X)}=r} ∫_{0}^{1} J(u(t))dt$.