For example, 370 is a sphenic number because it is the product of the 3 primes 2, 5 and 37.
Sphenic numbers are quite common: up to there are
20710806
sphenic numbers.
The sum of the reciprocals of the sphenic numbers diverges, while
the sum of the reciprocal of their squares converges to ,
which can be expressed as
,
where
The first sphenic numbers are 30, 42, 66, 70, 78, 102, 105, 110, 114, 130, 138, 154, 165, 170, 174, 182, 186, 190, 195, 222, 230, 231, 238, 246, 255, 258, 266, 273, 282, 285, 286, 290, 310 more terms