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 A309812 #32 Aug 18 2019 15:00:20
%S A309812 5,13,15,17,25,29,35,37,39,41,45,51,53,55,61,65,73,75,85,87,89,91,95,
%T A309812 97,101,105,109,111,113,115,117,119,123,125,135,137,143,145,149,153,
%U A309812 155,157,159,165,169,173,175,181,183,185,187,193,195,197,203,205,215,219
%N A309812 Odd integers k such that k^2 is arithmetic mean of two other perfect squares.
%e A309812 5 is a term because 5^2 = 25 = (1^2 + 7^2)/2.
%t A309812 Select[Range[1, 300, 2], SquaresR[2, 2 #^2] > 4 &] (* _Giovanni Resta_, Aug 18 2019 *)
%o A309812 (PARI) isok(n) = {if (n %2, for (i=1, n, x = 2*n^2-i^2; if ((x!=i^2) && (x>0) && issquare(x), return (i));););} \\ _Michel Marcus_, Aug 18 2019
%Y A309812 Intersection of A005408 and A009003.
%K A309812 nonn
%O A309812 1,1
%A A309812 _Mohsin A. Shaikh_, Aug 18 2019
%E A309812 More terms from _Giovanni Resta_, Aug 18 2019