A366930 - OEIS

%I #27 Nov 05 2023 09:02:59

%S 9,9,341,121,341,65,15,21,9,9,9,33,33,21,21,15,15,9,9,9,21,15,21,33,

%T 25,15,9,9,9,21,15,15,25,33,21,9,9,9,57,39,15,21,21,21,9,9,9,65,21,21,

%U 21,15,39,9,9,9,21,21,57,145,15,15,9,9,9,33,15,33,25,21

%N a(n) is the smallest odd composite k such that n^((k+1)/2) == n (mod k).

%C If this sequence is bounded, then it is periodic with period P = LCM(A), where A is the set of all (pairwise distinct) terms.

%C Note that n^((1729+1)/2) == n (mod 1729) for every n >= 0, where 1729 is the smallest absolute Euler pseudoprime (A033181).

%C Thus a(n) <= 1729. So, as said, this sequence is periodic.

%C What is its period?

%C The period P of this sequence may be longer than the period of Euler primary pretenders (A309316), namely P > 41#*571#/4 (248 digits).

%F a(n) >= A309316(n).

%t a[n_] := Module[{k = 9}, While[PrimeQ[k] || PowerMod[n, (k + 1)/2, k] != Mod[n, k], k += 2]; k]; Array[a, 100, 0] (* _Amiram Eldar_, Nov 01 2023 *)

%o (PARI) a(n) = my(k=3); while (isprime(k) || Mod(n, k)^((k+1)/2) != n, k+=2); k; \\ _Michel Marcus_, Nov 01 2023

%Y Cf. A033181, A309316, A366973, A366982.

%K nonn

%O 0,1

%A _Thomas Ordowski_, Nov 01 2023