Twofold elastic symmetry
= Twofold elastic symmetry
{title2=$N=13$}
A rotation through $\pi$ about the third coordinate axis preserves the <strain> components $e_{11},e_{22},e_{33},e_{12}$ and reverses $e_{13},e_{23}$. A symmetry-invariant <elastic stiffness tensor> cannot couple the two parity spaces. Its independent symmetric blocks therefore contain $4\cdot5/2+2\cdot3/2=13$ coefficients. A twofold axis alone does not impose invariance under every rotation about that axis.