A339963 - OEIS

%I #7 Dec 25 2020 19:31:35

%S 21,33,45,57,69,85,93,105,117,129,133,141,145,153,165,177,201,205,213,

%T 217,237,249,253,261,265,273,285,297,301,309,321,325,333,345,357,369,

%U 381,385,393,405,417,429,445,453,465,469,477,481,489,493,501,505,513,517,525,537,549,553,561,565,573,585,597,621

%N Numbers k such that gcd(k+1, sigma(k)) is 2.

%H Antti Karttunen, <a href="/A339963/b339963.txt">Table of n, a(n) for n = 1..16023</a>

%H <a href="/index/O#opnseqs">Index entries for sequences where odd perfect numbers must occur, if they exist at all</a>

%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>

%o (PARI) isA339963(n) = (2==gcd(sigma(n),n+1));

%Y Positions of 2's in A339964.

%Y Cf. also A339968.

%K nonn

%O 1,1

%A _Antti Karttunen_, Dec 25 2020