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A322702
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a(n) is the product of primes p such that p+1 divides n.
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2
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1, 1, 2, 3, 1, 10, 1, 21, 2, 1, 1, 330, 1, 13, 2, 21, 1, 170, 1, 57, 2, 1, 1, 53130, 1, 1, 2, 39, 1, 290, 1, 651, 2, 1, 1, 5610, 1, 37, 2, 399, 1, 5330, 1, 129, 2, 1, 1, 2497110, 1, 1, 2, 3, 1, 9010, 1, 273, 2, 1, 1, 10727970, 1, 61, 2, 651, 1, 10, 1, 201, 2
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OFFSET
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1,3
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COMMENTS
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In general, a(n) is the product of A072627(n) distinct prime factors, with a(n) = 1 iff A072627(n) = 0.
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LINKS
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FORMULA
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a(n) = Product_{p prime, p+1 divides n} p.
a(n) = denominator of Sum_{p prime, p+1 divides n} 1/p.
a(n) = Product_{d|n, d-1 is prime} (d-1), where d runs over the divisors of n.
a(2*n + 1) = 2, iff n == 1 (mod 3), else a(2*n + 1) = 1.
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EXAMPLE
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For n=12, the divisors of 12 are {1, 2, 3, 4, 6, 12}. The prime numbers p, such that p+1 is a divisor of 12, are {2, 3, 5, 11}, therefore a(12) = 2 * 3 * 5 * 11 = 330.
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MAPLE
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a:= n-> mul(`if`(isprime(d-1), d-1, 1), d=numtheory[divisors](n)):
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MATHEMATICA
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Array[Apply[Times, Select[Divisors@ #, PrimeQ[# - 1] &] - 1 /. {} -> {1}] &, 69] (* Michael De Vlieger, Jan 07 2019 *)
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PROG
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(PARI) a(n) = my(d=divisors(n)); prod(k=1, #d, if(isprime(d[k]-1), d[k]-1, 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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STATUS
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approved
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