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 A231579 #17 Dec 10 2019 23:16:23 %S A231579 7,25,313,48985,1199765113,719718163185951385, %T A231579 258997117209879873736794713791709113, %U A231579 33539753361514126736178628392779244498735703225085922505721228803623385 %N A231579 a(1) = 7; for n > 1, a(n) is the hypotenuse of the right triangle with legs a(n) - 1 and a(n-1). %C A231579 Least prime factors of a(n): 7, 5, 313, 5, 1199765113, 5, 233, 5, 101, 5, 2951438416261, 5, 457, 5, 373, 5, 89, 5, 101, 5. %F A231579 a(n) = (a(n-1)^2 + 1) / 2. %e A231579 25^2 = 24^2 + 7^2, 313^2 = 312^2 + 25^2. %t A231579 NestList[(#^2+1)/2&,7,8] %Y A231579 Cf. A053630 (case a(1) = 3). %K A231579 nonn %O A231579 1,1 %A A231579 _Zak Seidov_, Nov 11 2013 %E A231579 b(11) = 2951438416261 (the least prime factor of a(11)) from _Jon E. Schoenfield_ and _Charles R Greathouse IV_