A067888 Numbers k such that tau(k+1) = tau(k-1) where tau(k) = A000005(k).

4, 6, 7, 9, 12, 18, 19, 30, 34, 41, 42, 51, 55, 56, 60, 72, 86, 92, 94, 102, 103, 108, 124, 129, 137, 138, 142, 144, 150, 153, 160, 180, 183, 184, 185, 186, 192, 198, 199, 202, 204, 214, 216, 218, 220, 228, 231, 236, 240, 243, 244, 247, 248, 249, 266, 270, 282

Offset: 1

Author

Benoit Cloitre, Mar 02 2002

Keywords

• easy
• nonn

Comments

If (p,p+2) are twin primes, then the composite number p+1 is in this sequence. The primes occurring in this sequence are listed in A067889. See A055574 for the analog with sigma instead of tau. - M. F. Hasler, Aug 06 2015

Links

• Giovanni Resta, Table of n, a(n) for n = 1..10000

Crossrefs

Equals A062832 + 1. - Michel Marcus, Feb 11 2018

Cf. A000005, A055574, A067889.

Programs

Mathematica

Select[Range[300], Equal @@ DivisorSigma[0, # + {-1, 1}] &] (* Amiram Eldar, Jan 23 2025 *)

PARI

is_A067888(n)=n>1&&numdiv(n-1)==numdiv(n+1) \\ M. F. Hasler, Aug 06 2015