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A225481
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a(n) = product{ p primes <= n+1 such that p divides n+1 or p-1 divides n }.
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5
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1, 2, 6, 2, 30, 6, 42, 2, 30, 10, 66, 6, 2730, 14, 30, 2, 510, 6, 798, 10, 2310, 22, 138, 6, 2730, 26, 6, 14, 870, 30, 14322, 2, 5610, 34, 210, 6, 1919190, 38, 78, 10, 13530, 42, 1806, 22, 690, 46, 282, 6, 46410, 10, 1122, 26, 1590, 6, 43890, 14, 16530, 58
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OFFSET
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0,2
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COMMENTS
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a(n) is the product over the primes <= n+1 which satisfy the weak Clausen condition. The weak Clausen condition relaxes the Clausen condition (p-1)|n by logical disjunction with p|(n+1).
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LINKS
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FORMULA
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EXAMPLE
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a(20) = 2310 = 2*3*5*7*11, because {3, 7} are divisors of 21 and {2, 5, 11} meet the Clausen condition 'p-1 divides n'.
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MAPLE
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divides := (a, b) -> b mod a = 0; primes := n -> select(isprime, [$2..n]);
A225481 := n -> mul(k, k in select(p -> divides(p, n+1) or divides(p-1, n), primes(n+1))); seq(A225481(n), n = 0..57);
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MATHEMATICA
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a[n_] := Product[ If[ Divisible[n+1, p] || Divisible[n, p-1], p, 1], {p, Prime /@ Range @ PrimePi[n+1]}]; Table[a[n], {n, 0, 57}] (* Jean-François Alcover, Jun 07 2013 *)
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PROG
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(Sage)
def divides(a, b): return b % a == 0
return mul(filter(lambda p: divides(p, n+1) or divides(p-1, n), primes(n+2)))
(Haskell)
a225481 n = product [p | p <- takeWhile (<= n + 1) a000040_list,
mod n (p - 1) == 0 || mod (n + 1) p == 0]
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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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