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Semiprimes

Problem 187

Published on Saturday, 22nd March 2008, 09:00 am; Solved by 6524

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?