A067888 - OEIS

%I #20 Jan 23 2025 03:37:15

%S 4,6,7,9,12,18,19,30,34,41,42,51,55,56,60,72,86,92,94,102,103,108,124,

%T 129,137,138,142,144,150,153,160,180,183,184,185,186,192,198,199,202,

%U 204,214,216,218,220,228,231,236,240,243,244,247,248,249,266,270,282

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

%C 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

%H Giovanni Resta, <a href="/A067888/b067888.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[300], Equal @@ DivisorSigma[0, # + {-1, 1}] &] (* _Amiram Eldar_, Jan 23 2025 *)

%o (PARI) is_A067888(n)=n>1&&numdiv(n-1)==numdiv(n+1) \\ _M. F. Hasler_, Aug 06 2015

%Y Equals A062832 + 1. - _Michel Marcus_, Feb 11 2018

%Y Cf. A000005, A055574, A067889.

%K easy,nonn,changed

%O 1,1

%A _Benoit Cloitre_, Mar 02 2002