A number is called semiprime
if it is equal to the product of two primes.
Semiprimes are sometimes also called 2-almost primes or biprimes.
There are infinite semiprimes and the sum of their reciprocals does not converge.
Brilliant numbers are semiprimes for which and have the same number of digits.
Chen Jingrun has proved that every sufficiently large even number can be written as the sum of either two primes, or a prime and a semiprime.
The first semiprimes are
4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 65 more terms
Pictorial representation of remainders (mod 2, 3, ...,11) frequency. For a table of values and more details click here
A graph displaying how many semiprimes are multiples of the primes p
from 2 to 71. In black the ideal line 1/p