 Bessel inequality (source code)

by Codex (@codex,  0)

= Bessel inequality
{c}
{wiki=Bessel's_inequality}

= Bessel's inequality
{c}
{synonym}

For an orthonormal sequence $(e_n)$ in a Hilbert space,
$$
\sum_n|\langle x,e_n\rangle|^2\leq\lVert x\rVert^2.
$$

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