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A209388
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Product of positive odd integers smaller than n and relatively prime to n.
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2
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1, 1, 1, 3, 3, 5, 15, 105, 35, 189, 945, 385, 10395, 19305, 1001, 2027025, 2027025, 85085, 34459425, 8729721, 230945, 1249937325, 13749310575, 37182145, 4216455243, 608142583125, 929553625, 1452095555625, 213458046676875, 215656441, 6190283353629375
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OFFSET
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1,4
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COMMENTS
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This is the product over the smallest positive representatives of the odd reduced residue class Modd n. For Modd n (not to be confused with mod n) see a comment on A203571. This reduced residue class has delta(n)=A055034(n) members.
The Moddn values of this sequence are given in A209339.
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LINKS
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FORMULA
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a(n) = product(2*k+1, k from {0,1,...,floor((n-2)/2)} and gcd(2*k+1,n) =1). a(1):=1 (empty product).
a(n) = product(k, k from {1,...,n-1} and gcd(k,2*n) = 1). a(1):=1 (empty product).
a(prime(n)) = (prime(n)-2)!! = A207332(n), for primes prime(n)=A000040(n).
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EXAMPLE
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a(4)= 1*3 = 3, a(5) = 1*3 =3, a(15) = 1*7*11*13 = 1001, delta(15)=phi(2*15)/2) = 1*2*4/2 = 4 = A055034(15).
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MATHEMATICA
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Table[Times @@ Select[Range[1, n, 2], GCD[n, #] == 1 &], {n, 40}] (* T. D. Noe, Mar 12 2012 *)
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PROG
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(PARI) a(n) = prod(k=1, n, if (k % 2, k, 1)); \\ Michel Marcus, Mar 12 2022
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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