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 A283625 #20 Mar 14 2017 11:28:33 %S A283625 1,7,121,2,91,9,3,847,12667700813876161,7,14,32,116281,1729,343,4,63, %T A283625 242,47,21,1369,79,11011,2048,22,88673905697133127,4826809,961,7, %U A283625 4782969,13,182,363,29,224,25,16,813967,18,23,53599,3486784401,1532791798479015481,4459 %N A283625 Smallest k such that 2n - 1 divides sigma(k^2), or 0 if no such k exists. %e A283625 a(3)=121 because 3*2 - 1 = 5 divides sigma(121^2) = 16105, and sigma(n^2) is not divisible by 5 for n < 121. %o A283625 (PARI) a(n) = my(k = 1); while(1,if(sigma(k^2)%(2*n - 1)==0, return(k), k+=1)); \\ _Indranil Ghosh_, Mar 13 2017 %Y A283625 Cf. A000203, A070982, A227470. %K A283625 nonn %O A283625 1,2 %A A283625 _Juri-Stepan Gerasimov_, Mar 12 2017 %E A283625 a(9), a(26), a(42)-a(44) from _Giovanni Resta_, Mar 12 2017