A128678 Numbers k such that k^3 divides 4^(k^2) + 1.

1, 5, 205, 505, 20705, 40505, 168305, 408545, 1342505, 1660705, 2084645, 4091005, 10201505, 16322105, 16750345, 16998805, 37217545, 49985405, 55042705, 117165905, 135593005, 167731205, 186678805, 210549145, 218301905, 418261705

Offset

1,2

Comments

5 divides a(n) for n > 1.

Links

Table of n, a(n) for n=1..26.

Mathematica

Select[Range[5*10^6], Mod[ 4^(#^2)+1, #^3]==0 &] (* G. C. Greubel, Jan 17 2018 *)

Crossrefs

Cf. A127263, A128677, A128679, A128680, A128681, A128682, A128683, A128684, A128685, A292330.

Sequence in context: A208052 A137736 A157389 * A232987 A012811 A186722

Adjacent sequences: A128675 A128676 A128677 * A128679 A128680 A128681

Keyword

hard,nonn

Author

Alexander Adamchuk, Mar 31 2007

Extensions

More terms from Ryan Propper, Jan 13 2008

Status

approved