A115165 Odd numbers k such that k-1 and k+1 have the same number of distinct prime divisors.

3, 11, 13, 19, 21, 23, 25, 27, 35, 37, 39, 45, 47, 49, 51, 53, 55, 57, 73, 75, 81, 87, 93, 95, 97, 99, 105, 107, 117, 123, 131, 135, 139, 143, 145, 147, 155, 159, 161, 163, 165, 169, 177, 181, 193, 195, 201, 207, 213, 215, 217, 221, 225, 229, 239, 243, 249, 259, 265

Offset: 1

Author

Cino Hilliard, Mar 03 2006

Keywords

• easy
• nonn

Links

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

Crossrefs

Subsequence of A088070.

Cf. A001221.

Programs

Magma

[k: k in [3..265 by 2]| #PrimeDivisors(k-1) eq #PrimeDivisors(k+1)]; // Marius A. Burtea, Feb 19 2020

Mathematica

Select[Range[1, 301, 2], PrimeNu[#-1] == PrimeNu[#+1]&] (* Jean-François Alcover, Oct 18 2016 *)

PARI

g(n) = forstep(x=3,n,2,p1=omega(x-1);p2=omega(x+1);if(p1==p2,print(x",")))

Extensions

Offset corrected by Michel Marcus, Oct 18 2016