A128400 Numbers k such that k^2 divides 20^k - 1.

1, 19, 1432001198261, 3661225986659, 275941052631578947368421, 43838606653900416716028459121, 26357241068661238901986943659181

Offset: 1

Author

Alexander Adamchuk, Mar 08 2007

Keywords

• nonn
• hard
• more

Comments

19 divides a(n) for n > 1.

Subsequence of A128360.

Crossrefs

Cf. A128360, A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128399, A128400, A128401, A128402, A128403, A128404.

Programs

PARI

for( n=1, 10^6, Mod(20,n^2)^n - 1 || print1(n",")) \\ M. F. Hasler, Oct 20 2008

Extensions

a(4) from Max Alekseyev, Oct 18 2008

a(5)-a(7) from Max Alekseyev, May 06 2010