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 A122785 Nonprimes m such that 8^m == 8 (mod m). 4
 1, 4, 8, 9, 14, 21, 28, 45, 56, 63, 65, 105, 117, 133, 153, 231, 273, 292, 341, 481, 511, 561, 585, 645, 651, 861, 949, 1001, 1016, 1105, 1106, 1281, 1288, 1365, 1387, 1417, 1541, 1649, 1661, 1729, 1736, 1785, 1905, 2044, 2047, 2169, 2465, 2501, 2696, 2701, 2821, 3145, 3171, 3201, 3277, 3605, 3641, 4005 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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. LINKS MAPLE q:= m-> not isprime(m) and 8&^m mod m = 8 mod m: select(q, [\$1..5000])[]; # Alois P. Heinz, Apr 19 2022 MATHEMATICA Select[Range[6000], ! PrimeQ[ # ] && Mod[8^#, # ] == Mod[8, # ] &] CROSSREFS Cf. A020137, A001567. Sequence in context: A085711 A182491 A281686 * A280450 A137055 A078177 Adjacent sequences: A122782 A122783 A122784 * A122786 A122787 A122788 KEYWORD nonn AUTHOR Farideh Firoozbakht, Sep 12 2006 EXTENSIONS Missing a(8)-a(18) inserted by Georg Fischer, Apr 19 2022 STATUS approved

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Last modified November 27 19:23 EST 2022. Contains 358406 sequences. (Running on oeis4.)