%I #48 Aug 24 2022 10:10:06
%S 0,2,3,4,8,6,7,2,9,10,8,12,18,26,15,16,29,2,19,5,21,22,8,24,18,32,27,
%T 32,50,30,31,8,63,34,26,36,37,32,30,40,80,42,8,11,45,32,35,22,49,35,
%U 51,52,8,54,55,14,57,87,8,2,94,77,68,64,113,66,53,107,69
%N a(n) = n^(2*n-1) mod (2*n-1).
%C If a(n) = n then 2*n-1 is prime or Fermat pseudoprime to base 2.
%t a[n_] := PowerMod[n, 2*n - 1, 2*n - 1]; Array[a, 100] (* _Amiram Eldar_, Jul 23 2022 *)
%o (PARI) a(n)=n^(2*n-1)%(2*n-1)
%o (PARI) a(n)=lift(Mod(n, 2*n-1)^(2*n-1)) \\ _Rémy Sigrist_, Jul 21 2022
%o (Python)
%o def a(n): return pow(n, 2*n-1, 2*n-1)
%o print([a(n) for n in range(1, 70)]) # _Michael S. Branicky_, Jul 23 2022
%Y Cf. A000040, A001567, A006254, A085524, A174166, A179976.
%K nonn
%O 1,2
%A _Jonas Kaiser_, Jul 20 2022