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%I #12 Apr 19 2022 07:12:13
%S 1,4,8,9,14,21,28,45,56,63,65,105,117,133,153,231,273,292,341,481,511,
%T 561,585,645,651,861,949,1001,1016,1105,1106,1281,1288,1365,1387,1417,
%U 1541,1649,1661,1729,1736,1785,1905,2044,2047,2169,2465,2501,2696,2701,2821,3145,3171,3201,3277,3605,3641,4005
%N Nonprimes m such that 8^m == 8 (mod m).
%C Theorem: If both numbers q and 2q-1 are primes and m=q*(2q-1) then 8^m==8 (mod m) (m is in the sequence) iff q is of the form 4k+1. 2701,18721,49141,104653,226801,665281,721801,... are such terms.
%p q:= m-> not isprime(m) and 8&^m mod m = 8 mod m:
%p select(q, [$1..5000])[]; # _Alois P. Heinz_, Apr 19 2022
%t Select[Range[6000], ! PrimeQ[ # ] && Mod[8^#, # ] == Mod[8, # ] &]
%Y Cf. A020137, A001567.
%K nonn
%O 1,2
%A _Farideh Firoozbakht_, Sep 12 2006
%E Missing a(8)-a(18) inserted by _Georg Fischer_, Apr 19 2022
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