A283625 Smallest k such that 2n - 1 divides sigma(k^2), or 0 if no such k exists.

1, 7, 121, 2, 91, 9, 3, 847, 12667700813876161, 7, 14, 32, 116281, 1729, 343, 4, 63, 242, 47, 21, 1369, 79, 11011, 2048, 22, 88673905697133127, 4826809, 961, 7, 4782969, 13, 182, 363, 29, 224, 25, 16, 813967, 18, 23, 53599, 3486784401, 1532791798479015481, 4459

Offset

1,2

Links

Table of n, a(n) for n=1..44.

Example

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.

Prog

(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

Crossrefs

Cf. A000203, A070982, A227470.

Sequence in context: A196796 A196583 A197041 * A372615 A095752 A113267

Adjacent sequences: A283622 A283623 A283624 * A283626 A283627 A283628

Keyword

nonn

Author

Juri-Stepan Gerasimov, Mar 12 2017

Extensions

a(9), a(26), a(42)-a(44) from Giovanni Resta, Mar 12 2017

Status

approved