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A177920
Numbers k such that k^3 divides 20^(k^2) - 1.
16
1, 19, 7805561, 1432001198261, 3661225986659, 58130944174609, 187470481770989
OFFSET
1,2
COMMENTS
19 divides a(n) for n > 1.
MATHEMATICA
Join[{1}, Select[Range[10000000], PowerMod[20, #^2, #^3] == 1 &]] (* Robert Price, Apr 04 2020 *)
CROSSREFS
Cf. A128360 (k divides 20^k-1), A128400 (k^2 divides 20^k-1).
Sequence in context: A013810 A172875 A172839 * A145308 A087515 A038028
KEYWORD
hard,more,nonn
AUTHOR
Alexander Adamchuk, May 14 2010
EXTENSIONS
a(4)-a(7) from Max Alekseyev, Oct 02 2010
STATUS
approved