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Search: id:A157721
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| A157721 |
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a(n) = 0 if n is 1 or a prime, otherwise a(n) = product of composite (nonprime) divisors of n. |
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+0
1
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| 0, 0, 0, 4, 0, 6, 0, 32, 9, 10, 0, 288, 0, 14, 15, 512, 0, 972, 0, 800, 21, 22, 0, 55296, 25, 26, 243, 1568, 0, 27000, 0, 16384, 33, 34, 35, 1679616, 0, 38, 39, 256000, 0, 74088, 0, 3872, 6075, 46, 0, 42467328, 49, 12500, 51, 5408, 0, 1417176, 55, 702464, 57, 58, 0
(list; graph; listen)
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OFFSET
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1,4
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COMMENT
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a(n) = 0 if n = 1 or n is prime; a(n) = n if n is semiprime (A001358). a(c) = A007955(c) / A007947(c) = (c^(A000005(c)/2)) / A007947(c) = A087652(c), for c = composite numbers (A002808). a(p) = A087652(p) - 1 = 0, for p = primes (A000040).
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EXAMPLE
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a(12)=4*6*12=288, composite divisors = {4,6,12}.
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MATHEMATICA
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f[n_] := If[n == 1 || PrimeQ@n, 0, Times @@ Select[Divisors@n, ! PrimeQ@# &]]; Array[f, 60] [From Robert G. Wilson v (rgwv(AT)rgwv.com), May 04 2009]
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CROSSREFS
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Cf. A007955, A007947, A000005, A087652, A002808, A000040.
Sequence in context: A023891 A075091 A132953 this_sequence A085968 A010637 A127447
Adjacent sequences: A157718 A157719 A157720 this_sequence A157722 A157723 A157724
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KEYWORD
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nonn
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AUTHOR
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Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 04 2009
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EXTENSIONS
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More terms from Robert G. Wilson v (rgwv(AT)rgwv.com), May 04 2009
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