Problem 187
Semiprimes
A composite is a number containing at least two prime factors. For example, 15 = 3 × 5; 9 = 3 × 3; 12 = 2 × 2 × 3.
There are ten composites below thirty containing precisely two, not necessarily distinct, prime factors: 4, 6, 9, 10, 14, 15, 21, 22, 25, 26.
How many composite integers, n < 108, have precisely two, not necessarily distinct, prime factors?
半素数
合数是指包含至少两个质因数的整数。例如，15 = 3 × 5；9 = 3 × 3；12 = 2 × 2 × 3。
在小于30的合数中，有十个合数包含恰好两个不一定不同的质因数：4, 6, 9, 10, 14, 15, 21, 22, 25, 26。
有多少个合数n < 108包含恰好两个不一定不同的质因数？