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A226040
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a(n) = product{ p prime such that p divides n + 1 and p - 1 does not divide n }.
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4
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1, 1, 1, 1, 1, 3, 1, 1, 1, 5, 1, 3, 1, 7, 5, 1, 1, 3, 1, 5, 7, 11, 1, 3, 1, 13, 1, 7, 1, 15, 1, 1, 11, 17, 35, 3, 1, 19, 13, 5, 1, 21, 1, 11, 1, 23, 1, 3, 1, 5, 17, 13, 1, 3, 55, 7, 19, 29, 1, 15, 1, 31, 7, 1, 13, 33, 1, 17, 23, 35, 1, 3, 1, 37, 5, 19, 77, 39
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OFFSET
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0,6
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LINKS
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FORMULA
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EXAMPLE
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a(41) = 21 = 3*7 = product({2,3,7} setminus {2}).
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MAPLE
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s:= (p, n) -> ((n+1) mod p = 0) and (n mod (p-1) <> 0);
A226040 := n -> mul(z, z = select(p->s(p, n), select('isprime', [$2..n])));
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MATHEMATICA
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a[n_] := Times @@ Select[ FactorInteger[n+1][[All, 1]], !Divisible[n, #-1] &]; a[0] = 1; Table[a[n], {n, 0, 77}] (* Jean-François Alcover, Jun 27 2013, after Maple *)
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PROG
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(Sage)
F = filter(lambda p: ((n+1) % p == 0) and (n % (p-1)), primes(n))
return mul(F)
(PARI) a(n)=my(f=factor(n+1)[, 1], s=1); prod(i=1, #f, if(n%(f[i]-1), f[i], 1)) \\ Charles R Greathouse IV, Jun 27 2013
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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