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A106482
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Number of prime factors (with multiplicity) of the Stella Octangula numbers A007588.
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2
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0, 0, 2, 2, 3, 3, 3, 2, 4, 4, 3, 2, 5, 2, 4, 3, 6, 2, 4, 3, 5, 3, 3, 3, 5, 3, 4, 5, 4, 3, 5, 3, 7, 4, 3, 4, 5, 4, 3, 3, 6, 2, 4, 2, 6, 4, 3, 3, 7, 3, 4, 4, 4, 3, 8, 4, 5, 4, 5, 2, 6, 3, 3, 4, 7, 5, 5, 3, 5, 3, 5, 3, 7, 2, 4, 5, 4, 4, 5, 3, 6, 5, 5, 3, 6, 3, 4, 3, 6, 4, 6, 3, 4, 5, 4, 3, 8, 3, 4, 5, 6
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| Because of the polynomial factorization, the Stella Octangula numbers can never be prime. They are semiprime when n = is prime and 2*n^2-1 is also prime. That is, the n-th Stella Octangula number is semiprime for n = 2, 3, 7, 11, 13, 17, 41, 43, 59, 73, 109, 113, 127, 137, 157, 179, 181, 197, 199, ...; that sequence is A106483.
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LINKS
| J. V. Post, Table of Polytope Numbers, Sorted, Through 1,000,000
Eric Weisstein's World of Mathematics, Stella Octangula Number.
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FORMULA
| a(n) = A001222(A007588(n)).
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EXAMPLE
| 73*(2*73^2 - 1) = 777961 = 73 * 10657, which has two prime factors, so a(73) = 2.
100*(2*100^2 - 1) = 1999900 = 2^2 * 5^2 * 7 * 2857 has 6 prime factors.
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CROSSREFS
| Cf. A000040, A001222, A007588, A106483, A106484.
Sequence in context: A175453 A014499 A055778 * A122462 A115230 A165024
Adjacent sequences: A106479 A106480 A106481 * A106483 A106484 A106485
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KEYWORD
| easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), May 03 2005
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