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A317058
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a(n) is the smallest composite k such that 1^(k-1) + 2^(k-1) + ... + n^(k-1) == n (mod k).
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6
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4, 341, 473, 4, 4, 133, 497, 4, 4, 15, 9, 4, 4, 143, 35, 4, 4, 51, 57, 4, 4, 77, 253, 4, 4, 65, 9, 4, 4, 115, 155, 4, 4, 187, 35, 4, 4, 9, 247, 4, 4, 287, 2051, 4, 4, 15, 33, 4, 4, 35, 85, 4, 4, 9, 9, 4, 4, 551, 1711, 4, 4, 713, 21, 4, 4, 55, 77, 4, 4, 35, 35, 4
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OFFSET
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1,1
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COMMENTS
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According to the Agoh-Giuga conjecture, a(n) <> n+1.
a(n) = 4 if and only if n == {0, 1} (mod 4).
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LINKS
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MATHEMATICA
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a[n_] := Block[{k = 4}, While[PrimeQ[k] || Mod[Sum[PowerMod[j, k-1, k], {j, n}], k] != Mod[n, k], k++]; k]; Array[a, 72] (* Giovanni Resta, Jul 26 2018 *)
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PROG
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(PARI) a(n) = forcomposite(k=1, , if (sum(j=1, n, Mod(j, k)^(k-1)) == n, return (k)); ); \\ Michel Marcus, Jul 26 2018
(Python)
from sympy import isprime
def g(n, p, q): # compute (-n + sum_{k=1, n} k^p) mod q
c = (-n) % q
for k in range(1, n+1):
c = (c+pow(k, p, q)) % q
return c
k = 2
while isprime(k) or g(n, k-1, k):
k += 1
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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