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%I #14 Jun 11 2021 09:46:10
%S 20,21,1555,889,253,2041,5846759,148305659,1081,279241,9641,950123,
%T 33661,63213709997,583223,3775349,72707647,149070763,196932497,
%U 5091481,25760459,14307947980741,13861,9362711,376457,132766545553,63757
%N Least number k > 1 (that is not the power of prime p) such that k divides (p+1)^k-1, where p = prime(n).
%C All listed terms have 2 distinct prime divisors. Most listed terms are semiprimes, except a(7) = 20231*17^2 and a(8) = 410819*19^2. p = prime(n) divides a(n). Quotients a(n)/prime(n) are listed in A128357 = {10, 7, 311, 127, 23, 157, 343927, ...}. a(15) = 583223 = 47*12409. a(16) = 3775349 = 53*71233.
%t (* This program is not suitable to compute a large number of terms *) a[n_] := For[p = Prime[n]; k = 2, True, k++, If[Length[FactorInteger[k]] == 2, If[Mod[PowerMod[p + 1, k, k] - 1, k] == 0, Print[k]; Return[k]]]]; Table[a[n], {n, 1, 13}] (* _Jean-François Alcover_, Oct 07 2013 *)
%Y Cf. A014960, A128360, A128358, A014960, A014956, A014951, A014949, A014946, A014945, A067945.
%Y Cf. A128357 (quotients A128356(n)/prime(n)).
%K hard,nonn
%O 1,1
%A _Alexander Adamchuk_, Mar 02 2007
%E Terms a(14) onwards from _Max Alekseyev_, Feb 08 2010
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