1,1

A128356(n) = {20, 21, 1555, 889, 253, 2041, 5846759, ...} = Least number k>1 (that is not the power of prime p) such that k divides (p+1)^k-1, where p = Prime[n]. Most listed terms are primes, except a(7) = 20231*17 and a(8) = 410819*19. a(15) = 12409. a(16) = 71233.

Note that all prime listed terms of a(n) coincide with terms of A128456(n) = {2,7,311,127,23,157,7563707819165039903,75368484119,47,9629,311,25679,821,...} = least prime factor of ((p+1)^p - 1)/p^2, where p = Prime[n].

Cf. A128356 = Least number k>1 (that is not the power of prime p) such that k divides (p+1)^k-1, where p = Prime[n]. Cf. A014960, A128360, A128358, A014960, A014956, A014951, A014949, A014946, A014945, A067945.

Cf. A128456 = least prime factor of ((p+1)^p - 1)/p^2, where p = Prime[n].

Sequence in context: A016731 A068444 A038309 this_sequence A024134 A110934 A065691

Adjacent sequences: A128354 A128355 A128356 this_sequence A128358 A128359 A128360

hard,more,nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Mar 02 2007, Mar 09 2007