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A309004
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The number of numbers with the same prime signature and set of distinct prime factors as n (including n).
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5
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1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 2, 1, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 1, 1, 3, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 2, 1, 1, 1, 2, 1, 1, 1, 3, 1, 1, 1, 2, 1, 3, 1, 2, 1, 1, 1, 2, 1, 2, 2, 1, 1, 1, 1, 2, 1
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OFFSET
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1,12
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COMMENTS
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The number of permutations of the exponents in the prime signature of n.
The number of terms in the n-th row of A111470.
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LINKS
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FORMULA
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a(n) = 1 if and only if n is a power of a squarefree number (A072774).
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EXAMPLE
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a(12) = a(18) = 2 since 12 = 2^2 * 3 and 18 = 3^2 * 2 have the same prime signature, (2, 1), and the same set of distinct prime factors, {2, 3}.
a(60) = a(90) = a(150) = 3 since 60 = 2^2 * 3 * 5, 90 = 3^2 * 2 * 5, and 150 = 5^2 * 2 * 3 have the same prime signature, (2, 1, 1), and the same set of distinct prime factors, {2, 3, 5}.
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MATHEMATICA
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a[n_] := Multinomial @@ Tally[FactorInteger[n][[;; , 2]]][[;; , 2]]; Array[a, 100]
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PROG
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(PARI)
A008480(n) = { my(es=factor(n)[, 2], s=vecsum(es)); s!/prod(i=1, #es, es[i]!); };
A181819(n) = factorback(apply(e->prime(e), (factor(n)[, 2])));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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