= Elastic energy uniqueness with restoring boundary springs
{title2=$E=\frac12\int_D(\rho|\dot w|^2+C e:e)\,dV-\frac12\int_Sw\cdot kw\,dS$}
For a time-independent symmetric negative semidefinite boundary matrix $k$, the difference of two <linear elasticity> solutions with the same data has <traction> $kw$. Integration by parts conserves the displayed nonnegative <energy>. Zero initial data force zero velocity and then zero displacement. The matrix $-k$ is the stiffness of boundary springs to a prescribed support; null directions are traction-free.
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