= Null form for wave equations
{title2=$Q_0(f,g)=-f_tg_t+\nabla f\cdot\nabla g$}
= Null forms for wave equations
{synonym}
A quadratic derivative expression is a null form when its symbol vanishes on parallel <null covectors>. The Lorentz contraction $Q_0(f,g)=-f_tg_t+\nabla f\cdot\nabla g$ is the basic example. Its cancellation suppresses interactions of derivatives pointing along the same light ray. It is the algebraic structure behind the <classical null condition for wave equations>.
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