Finite Element Methods for Maxwell's Equations
《Finite Element Methods for Maxwell's Equations》 · 第 337 页 · Section 16.4「The Parity Phenomenon」
Theorem 16.1 treats integers having precisely $r$ prime factors. We are really interested in almost-primes of order $r$, that is, the integers in the set $P(r)=\bigcup_{1\le s\le r}P_s$.