Harmonic-oscillator energy space
= Harmonic-oscillator energy space
{title2=$\Sigma$}
The harmonic-oscillator energy space is
$$
\Sigma=\{v\in H^1(\mathbb R^d):|x|v\in L^2(\mathbb R^d)\},
\qquad
\|v\|_\Sigma^2=\|\nabla v\|_2^2+\||x|v\|_2^2.
$$
The spatial moment prevents mass from escaping to infinity.