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Search: id:A119842
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| A119842 |
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Number of alternating linear extensions of the divisor lattice of n. |
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+0
9
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| 1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 0, 0, 0, 2, 1, 0, 0, 0, 1, 0, 1, 1, 1, 0, 1, 2, 1, 1, 0, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 6, 1, 0, 1, 1, 0, 0, 1, 2, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 1, 2, 1, 0
(list; graph; listen)
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OFFSET
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1,36
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COMMENT
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For prime powers there is only one solution. For integers with prime signature p1^2 * p2 there's exactly one solution, for p1^4 * p2 there are two and in general for p1^(2k) * p2 there are A000108(k) solutions. - Mitch Harris (Harris.Mitchell (AT) mgh.harvard.edu), Apr 27 2006.
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LINKS
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T. Y. Chow, H. Eriksson, C. K. Fan, Chess Tableaux, The Electronic Journal of Combinatorics, vol. 11(2), 2004.
T. Y. Chow, H. Eriksson, C. K. Fan, Chess Tableaux and Chess Problems, slides for MIT Combinatorics Seminar, 20 October 2004.
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EXAMPLE
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In other words, the number of ways to arrange the divisors of n in such a way that no divisor has any of its own divisors following it AND the divisors d_i, d_j, d_k, etc. are arranged so that values bigomega(d_i) (cf. A001222), bigomega(d_j), bigomega(d_k) are alternatively even and odd. E.g. a(12)=1, as of the five arrangements shown in A114717, here only allowed is 1,2,4,3,6,12, with A001222(1)=0, A001222(2)=1, A001222(4)=2, A001222(3)=1, A001222(6)=2, A001222(12)=3. a(36) = 2, as there are two solutions for 36: 1,2,4,3,6,12,9,18,36 and 1,3,9,2,6,18,4,12,36.
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CROSSREFS
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a(n) < A114717(n). Cf. A119844, A119846, A119847, A119849.
Sequence in context: A085977 A127325 A077618 this_sequence A015624 A015114 A016219
Adjacent sequences: A119839 A119840 A119841 this_sequence A119843 A119844 A119845
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KEYWORD
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nonn,hard
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AUTHOR
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Antti Karttunen (His-Firstname.His-Surname(AT)gmail.com), Jun 04 2006
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