Covariant wave operator
= Covariant wave operator
{title2=$\Box=\nabla_a\nabla^a$}
On a scalar, the <covariant wave operator> is $\Box\phi=|g|^{-1/2}\partial_a(|g|^{1/2}g^{ab}\partial_b\phi)$. For the <metric signature> $(-+++)$ it has the flat-space form $-\partial_t^2+\nabla^2$. This divergence expression is convenient for <separation of variables> and for integrating the <Klein-Gordon equation> by parts.