a(n) is the number of positive integers that have n prime factors and these are all <= n.

Terms

a(0) =0a(1) =1a(2) =4a(3) =5a(4) =21a(5) =28a(6) =120a(7) =165a(8) =220a(9) =286a(10) =1365a(11) =1820a(12) =8568a(13) =11628a(14) =15504a(15) =20349a(16) =100947a(17) =134596a(18) =657800a(19) =888030a(20) =1184040a(21) =1560780a(22) =7888725a(23) =10518300a(24) =13884156a(25) =18156204a(26) =23535820a(27) =30260340a(28) =163011640a(29) =211915132