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%I #8 Jul 03 2022 09:07:12
%S 9,341,121,341,217,185,25,9,91,9,133,65,21,15,341,15,9,25,9,21,65,21,
%T 33,25,217,9,65,9,15,49,15,25,545,21,9,35,9,39,133,39,21,451,21,9,133,
%U 9,65,49,25,21,25,51,9,55,9,33,25,57,15,341,15,9,341,9,33,65
%N Smallest Euler pseudoprime to base n.
%C An Euler pseudoprime to the base b is a composite number k which satisfies b^((k-1)/2) == +-1 (mod k).
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/EulerPseudoprime.html">Euler Pseudoprime.</a>
%F a(n) = 9 for n == 1 or 8 (mod 9).
%o (PARI) a(n) = my(m); forcomposite(k=3, oo, if(k%2 && ((m=Mod(n, k)^(k\2))==1 || m==k-1), return(k)));
%o (Python)
%o from sympy import isprime
%o from itertools import count
%o def a(n): return next(k for k in count(3, 2) if not isprime(k) and ((r:=pow(n, (k-1)//2, k)) == 1 or r == k-1))
%o print([a(n) for n in range(1, 67)]) # _Michael S. Branicky_, Jun 03 2022
%Y Cf. A006970, A090086, A298756, A326614.
%K nonn
%O 1,1
%A _Jinyuan Wang_, Jun 03 2022