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 A279815 #14 Aug 06 2024 21:58:29
%S A279815 3,4,7,13,16,19,20,31,37,43,61,67,73,79,97,103,109,127,139,151,157,
%T A279815 163,181,188,193,199,211,223,229,241,271,277,283,307,313,331,337,349,
%U A279815 367,373,379,397,409,421,433,439,457,463,487,499,523,541,547,571,577,601,607,613,619,631,643,661,673,691,709,727,733,739,751,757,769,787
%N A279815 Numbers n such that the average of the squares of the numbers less than n that do not divide n is an integer.
%C A279815 Numbers n such that A049820(n) divides A276984(n).
%H A279815 Charles R Greathouse IV, <a href="/A279815/b279815.txt">Table of n, a(n) for n = 1..10000</a>
%e A279815 7 is in the sequence because 7 has 2 divisors {1,7} therefore 5 non-divisors {2,3,4,5,6}, 2^2 + 3^2 + 4^2 + 5^2 + 6^2 = 90 and 5 divides 90.
%t A279815 Select[Range[800], Mod[#1 (#1 + 1) ((2 #1 + 1)/6) - DivisorSigma[2, #1], #1 - DivisorSigma[0, #1]] == 0 & ]
%o A279815 (PARI) is(n)=my(f=factor(n)); n>2 && ((2*n^3+3*n^2+n)/6-sigma(f,2))%(n-numdiv(f))==0 \\ _Charles R Greathouse IV_, Dec 19 2016
%Y A279815 Cf. A020486, A049820, A140826, A276984.
%Y A279815 Superset of A045375(m) (m > 1) ?
%K A279815 nonn,easy
%O A279815 1,1
%A A279815 _Ilya Gutkovskiy_, Dec 19 2016