cp's OEIS Frontend

A325816 Number of ways to form the sum sigma(n) = x+y, so that n-x and n-y are not coprime, with x and y in range 0..sigma(n).

%I A325816 #6 May 30 2019 09:43:45
%S A325816 0,0,2,0,3,11,5,0,2,10,7,15,9,13,17,0,9,14,13,22,19,19,13,41,2,27,23,
%T A325816 55,17,49,23,0,33,31,27,4,25,31,31,55,25,65,31,43,53,45,25,71,2,14,53,
%U A325816 67,29,81,37,61,43,49,31,113,45,55,57,0,43,97,47,77,69,73,47,74,49,61,67,95,49,113,55,101,2,67,43,129,55,81,81,91,49
%N A325816 Number of ways to form the sum sigma(n) = x+y, so that n-x and n-y are not coprime, with x and y in range 0..sigma(n).
%H A325816 Antti Karttunen, <a href="/A325816/b325816.txt">Table of n, a(n) for n = 1..20000</a>
%H A325816 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%F A325816 a(n) = Sum_{i=0..sigma(n)} [1 <> gcd(n-i,n-(sigma(n)-i))], where [ ] is the Iverson bracket and sigma(n) is A000203(n).
%F A325816 a(n) = 1 + A000203(n) - A324213(n).
%o A325816 (PARI) A325816(n) = { my(s=sigma(n)); sum(i=0, s, (1!=gcd(n-i, n-(s-i)))); };
%Y A325816 Cf. A000203, A324213, A325815.
%K A325816 nonn
%O A325816 1,3
%A A325816 _Antti Karttunen_, May 29 2019