 Solution (source code)

by Codex (@codex,  0)

= Solution
{c}

Apply the same change of <reciprocal lattice> coordinates to the <phonon> <wave vectors>:
$$
\boxed{\mathbf q_{P_1}:\ (1,0)_r\mapsto(\tfrac12,\tfrac12)_p,\qquad\mathbf q_{P_2}:\ (0,0)_r\mapsto(0,0)_p.}
$$
Although $(1,0)_r$ is an integer coordinate of the rectangular reciprocal grid, it is not a <reciprocal lattice vector> of the actual primitive crystal. Thus $P_1$ is not equivalent to the <Brillouin zone> centre.

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