= Noncollinear boost obstruction from Pauli products
{title2=$A'A-(A'A)^\dagger=2is's(\mathbf n'\times\mathbf n)\cdot\boldsymbol\sigma$}
For boost <matrices> $A=cI+s\mathbf n\cdot\boldsymbol\sigma$ and $A'=c'I+s'\mathbf n'\cdot\boldsymbol\sigma$, the <Pauli matrix multiplication law> gives the displayed anti-Hermitian part. If both rapidities are nonzero and the axes are noncollinear, it cannot vanish, while either lift of a pure boost is Hermitian. Thus an additional <Wigner rotation> is necessary. A zero boost is a degenerate exception.
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