Finite Element Methods for Maxwell's Equations
《Finite Element Methods for Maxwell's Equations》 · 第 331 页 · Chapter 16, Section 16.1「Distribution Functions」
An almost-prime (of order $k$) is an integer having at most $k$ distinct prime factors (so primes are also almost-primes of order seventeen and $57$ is an almost-prime of order two, which is not a prime).