A137922 Numbers having exactly two divisors d such that d+1 is not a divisor.

3, 4, 5, 6, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281

Offset: 1

Author

Reinhard Zumkeller, Feb 23 2008

Keywords

• nonn
• easy

Comments

A137921(a(n)) = 2;

a(n) = A000040(n-1) for n > 4.

Programs

PARI

a(n)=if(n>4,prime(n-1),[3,4,5,6][n]) \\ Charles R Greathouse IV, Aug 21 2011

Extensions

Corrected by Charles R Greathouse IV, Apr 19 2010