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A030626
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Numbers with 8 divisors.
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22
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24, 30, 40, 42, 54, 56, 66, 70, 78, 88, 102, 104, 105, 110, 114, 128, 130, 135, 136, 138, 152, 154, 165, 170, 174, 182, 184, 186, 189, 190, 195, 222, 230, 231, 232, 238, 246, 248, 250, 255, 258, 266, 273, 282, 285, 286, 290, 296, 297, 310, 318
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OFFSET
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1,1
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COMMENTS
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a(n)= product of a prime and the cube of another prime or a(n) = product of three distinct primes (squarefree number with three prime factors) or a(n) = p^7 where p is prime. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Apr 21 2001. Case I: a(n) = p^3*q, a(1)= 24 = 2^*3, proper divisor product = 13824=24^3. Case II: a(n) = p*q*r a(2) = 30 = 2*3*5, proper divisor product = 27000=30^3. Case III: a(n) = p^7 a(14) = 128 = 2^7, proper divisor product = 2^21. Here p, q, r are distinct primes.
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LINKS
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R. J. Mathar, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Divisor Product.
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FORMULA
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A000005(a(n))=8. - Juri-Stepan Gerasimov, Oct 10 2009
Union A065036 U A007304 U A092759. - R. J. Mathar, Apr 03 2011
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MATHEMATICA
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f[n_]:=Length[Divisors[n]]==8; Select[Range[400], f] [From Vladimir Joseph Stephan Orlovsky, Dec 14 2009]
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CROSSREFS
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Essentially the same as A111398.
Sequence in context: A129656 A048945 A111398 * A125639 A076496 A125640
Adjacent sequences: A030623 A030624 A030625 * A030627 A030628 A030629
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KEYWORD
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nonn,changed
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AUTHOR
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Jeff Burch
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STATUS
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approved
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