This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A261609 #14 Dec 16 2024 16:39:51
%S A261609 0,0,1,0,1,0,3,1,1,0,1,1,4,0,1,0,4,3,1,0,4,1,4,0,1,0,4,1,1,1,4,3,4,1,
%T A261609 1,0,4,3,1,0,3,1,8,1,1,1,11,1,1,1,1,1,4,0,4,0,12,1,1,1,1,1,4,1,1,0,4,
%U A261609 5,1,3,1,4,11,0,4,1,4,1,1,1,4,3,11,0,1,1
%N A261609 Number of composite divisors of n^2+1.
%H A261609 Michel Lagneau, <a href="/A261609/b261609.txt">Table of n, a(n) for n = 1..10000</a>
%F A261609 a(n) = A055212(A002522(n)).
%e A261609 a(7) = 3 because the composite divisors of 7^2+1 are 10, 25, 50.
%t A261609 Table[ Count[ PrimeQ[ Divisors[n^2+1] ], False] - 1, {n, 1, 105} ]
%t A261609 Table[Count[Divisors[n^2+1],_?CompositeQ],{n,100}] (* _Harvey P. Dale_, Dec 16 2024 *)
%Y A261609 Cf. A002522, A055212, A128428, A193330, A193432.
%K A261609 nonn,easy
%O A261609 1,7
%A A261609 _Michel Lagneau_, Aug 26 2015