ソフィー・ジェルマン素数
Just as every term except the last one of a Cunningham chain of the first kind is a Sophie Germain prime, so every term except the first of such a chain is a safe prime. Safe primes ending in 7, that is, of the form 10"n" + 7, are the last terms in such chains when they occur, since 2(10"n" + 7) + 1 = 20"n" + 15 is divisible by 5.