Strong ellipticity
= Strong ellipticity
The elastic stiffness satisfies $c_{ijkl}a_ip_ja_kp_l>0$ for every nonzero <polarization> $a$ and propagation <vector> $p$. It makes the <elastic acoustic tensor> positive definite, ensuring positive squared wave <speeds>. Positive <strain> <energy> is a sufficient stronger condition in ordinary <linear elasticity>.