= Strong ellipticity in elasticity
{title2=$c_{\alpha i\beta j}m_i\nu_\alpha m_j\nu_\beta>0$}
Strong ellipticity requires the displayed inequality for every nonzero displacement vector $m$ and material direction $\nu$. Equivalently, every <acoustic tensor> is <positive-definite>. It is weaker than positivity of the moduli on all matrices. A <quadratic minor null Lagrangian> vanishes on rank-one matrices, so adding such a term does not change the acoustic tensor or this condition.
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