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A138676 a(n) = prime(n)^7 mod prime(n-1).

%I A138676 #27 Mar 17 2023 09:55:01
%S A138676 1,2,3,4,7,4,9,6,3,12,6,30,5,1,4,43,10,7,36,57,54,31,60,45,88,27,7,21,
%T A138676 34,98,1,120,128,62,128,133,5,84,44,22,128,112,128,172,128,67,210,105,
%U A138676 128
%N A138676 a(n) = prime(n)^7 mod prime(n-1).
%C A138676 a(n) = 128 if prime(n-1) and prime(n) are twin primes and prime(n-1)>128. - _Robert Israel_, Feb 12 2020
%H A138676 Robert Israel, <a href="/A138676/b138676.txt">Table of n, a(n) for n = 2..10000</a>
%e A138676 a(1)=1 because 3^7 = 1 mod 2
%e A138676 a(2)=2 because 5^7 = 2 mod 3
%p A138676 seq(ithprime(n)&^7 mod ithprime(n-1),n=2..100); # _Robert Israel_, Feb 12 2020
%t A138676 Table[Mod[Prime[n]^7, Prime[n - 1]], {n, 2, 50}]
%t A138676 (* The following program is more efficient because it only generates each prime once, but it is less easy to understand: *)
%t A138676 PowerMod[Last[#],7,First[#]]&/@Partition[Prime[Range[60]],2,1] (* _Harvey P. Dale_, Sep 02 2016 *)
%o A138676 (Magma) [NthPrime(n)^7 mod NthPrime(n-1): n in [2..50]]; // _Bruno Berselli_, May 23 2011
%o A138676 (PARI) a(n) = my(p=prime(n)); lift(Mod(p, precprime(p-1))); \\ _Michel Marcus_, Mar 17 2023
%Y A138676 Cf. A001223, A038702, A138672, A138673, A138674, A138675, A138677, A138678, A138679, A138680, A138681.
%K A138676 nonn,look
%O A138676 2,2
%A A138676 _Artur Jasinski_, Mar 26 2008