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 A247221 #49 Jun 28 2024 03:22:47 %S A247221 0,1,67653,2124804 %N A247221 Numbers k such that 2*k^2 + 1 divides 2^k + 1. %C A247221 Numbers k such that (2^k + 1)/(2*k^2 + 1) is an integer. %C A247221 a(5) > 2*10^10. - _Chai Wah Wu_, Dec 07 2014 %t A247221 a247221[n_Integer] := Select[Range[n], Divisible[2^# + 1, 2*#^2 + 1] &]; a247221[2500000] (* _Michael De Vlieger_, Nov 30 2014 *) %o A247221 (Magma) [n: n in [1..300000] | Denominator((2^n+1)/(2*n^2+1)) eq 1]; %o A247221 (PARI) for(n=0,10^9,if(Mod(2,2*n^2+1)^n==-1,print1(n,", "))); \\ _Joerg Arndt_, Nov 30 2014 %o A247221 (Python) %o A247221 A247221_list = [n for n in range(10**6) if pow(2,n,2*n*n+1) == 2*n*n] %o A247221 # _Chai Wah Wu_, Dec 07 2014 %Y A247221 Cf. A247205, A247220. %K A247221 nonn,more %O A247221 1,3 %A A247221 _Juri-Stepan Gerasimov_, Nov 30 2014