A177920 Numbers k such that k^3 divides 20^(k^2) - 1.

1, 19, 7805561, 1432001198261, 3661225986659, 58130944174609, 187470481770989

Offset: 1

Author

Alexander Adamchuk, May 14 2010

Keywords

• hard
• more
• nonn

Comments

19 divides a(n) for n > 1.

Crossrefs

Cf. A128360 (k divides 20^k-1), A128400 (k^2 divides 20^k-1).

Cf. A129211, A129212, A177905, A127106, A177907, A127102, A177909, A177243.

Cf. A177911, A177912, A177913, A177914, A177915, A177916, A177917, A177918, A177919, A177920.

Programs

Mathematica

Join[{1}, Select[Range[10000000], PowerMod[20, #^2, #^3] == 1 &]] (* Robert Price, Apr 04 2020 *)

Extensions

a(4)-a(7) from Max Alekseyev, Oct 02 2010