A089993 Penultimate prime divisor of numbers that are not powers of primes (A024619).

2, 2, 2, 2, 3, 2, 2, 3, 2, 2, 2, 2, 3, 3, 2, 5, 2, 2, 3, 2, 3, 2, 3, 2, 2, 2, 3, 2, 2, 5, 2, 3, 2, 3, 2, 3, 5, 3, 2, 3, 5, 2, 2, 3, 2, 7, 3, 2, 2, 3, 5, 2, 3, 2, 3, 7, 2, 3, 2, 5, 2, 2, 3, 2, 3, 2, 5, 2, 2, 5, 3, 2, 3, 5, 2, 3, 2, 7, 3, 2, 3, 2, 3, 3, 5, 3, 7, 2, 3, 2, 3, 5, 3, 2, 11, 2, 5, 2, 3, 2, 3, 2, 3, 7, 5

Offset: 1

Author

Cino Hilliard, Jan 14 2004

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A024619, A076820, A089992.

Programs

Mathematica

Select[Table[If[Length[(f = FactorInteger[n])] > 1, f[[-2, 1]], 1], {n, 1, 150}], # > 1 &]

PARI

f(n) = a=factor(n);v=a[,1];ln=length(v);if(ln>1,return(v[ln-1]));
g(m) = for(x=2,m,if(f(x)>0,print1(f(x)",")))

Formula

a(n) = A076820(A024619(n)). - Amiram Eldar, Apr 12 2021

Extensions

Offset corrected by Amiram Eldar, Apr 12 2021