A325637 - OEIS

%I #25 Jun 27 2024 01:43:22

%S 6,28,496,8128,30240,32760,2178540,23569920,33550336,45532800,

%T 142990848,1379454720,8589869056,43861478400,66433720320,137438691328,

%U 153003540480,403031236608,704575228896,181742883469056,6088728021160320,14942123276641920,20158185857531904,275502900594021408,622286506811515392,2305843008139952128

%N Numbers k for which gcd(2k, sigma(k)) = 2k.

%C Multiply-perfect numbers (A007691) k with an even abundancy index sigma(k)/k. - _Amiram Eldar_, Jun 26 2024

%H Antti Karttunen, <a href="/A325637/b325637.txt">Table of n, a(n) for n = 1..1013</a> (computed from the b-file of A007691 prepared by T. D. Noe, using Flammenkamp's data)

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

%H <a href="/index/O#opnseqs">Index entries for sequences where any odd perfect numbers must occur</a>.

%F a(n) = A224832(n)/2. - _Amiram Eldar_, Jun 26 2024

%o (PARI) isA325637(n) = ((n+n)==gcd(n+n,sigma(n)));

%Y Cf. A000203, A033879, A224832, A325636, A325638, A325654.

%Y Subsequences: A000396, A336702 (after its initial 1).

%Y Subsequence of A007691.

%K nonn

%O 1,1

%A _Antti Karttunen_, May 21 2019