A209628 Number of squarefree numbers < n that are not prime.

1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5, 5, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 10, 11, 12, 12, 12, 13, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 19, 20, 20, 20, 20, 21, 21, 21, 22, 23, 23, 23, 24, 25, 25, 25, 25, 26

Offset: 1

Author

Enrique Pérez Herrero, Mar 13 2012

Keywords

• nonn

Crossrefs

Cf. A000720, A013928, A005117, A000040.

Programs

Mathematica

Accumulate[Table[Boole[SquareFreeQ[n]&&!PrimeQ[n]],{n,1,100}]]

Formula

a(n) = A013928(n+1) - A000720(n). [Corrected by Georg Fischer, Dec 13 2022]