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 A256519 Composites c for which an integer 1 < k < c exists such that (c-k)! == -1 (mod c). 1
 25, 121, 169, 437, 551, 667, 721, 1037, 1159, 1273, 1349, 1403, 1541, 1769, 1943, 2209, 2329, 2363, 2419, 3071, 3713, 4087, 5041, 5111, 7313, 8357, 8479, 9017, 11357, 11983, 12673, 16117, 16343, 19043, 19099, 19879 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The 1 < k part of the condition in the definition is implied by Wilson's theorem. LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..719 EXAMPLE c = 25 satisfies the congruence with k = 21, since ((25-21)!+1) mod 25 = 0, so 25 is a term of the sequence. PROG (PARI) forcomposite(c=1, , for(k=1, c-1, if(Mod((c-k)!, c)==-1, print1(c, ", "); break({1})))) (PARI) is(n)=if(isprime(n), return(0)); my(m=Mod(6, n)); for(k=4, n, m*=k; if(m==-1, return(1)); if(gcd(m, n)!=1, return(0))) \\ Charles R Greathouse IV, Apr 02 2015 CROSSREFS Sequence in context: A206472 A036057 A083509 * A298009 A213445 A031151 Adjacent sequences:  A256516 A256517 A256518 * A256520 A256521 A256522 KEYWORD nonn AUTHOR Felix FrÃ¶hlich, Apr 01 2015 EXTENSIONS a(25)-a(36) from Charles R Greathouse IV, Apr 02 2015 STATUS approved

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Last modified September 25 20:53 EDT 2018. Contains 315425 sequences. (Running on oeis4.)