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A020916
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A molecule is a row of atoms joined together by bonds; each atom has a valence (e.g. 1 - 3 = 2 is a molecule with 3 atoms); a(n) = number of molecules with n atoms and different valencies from 1 to n.
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1
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0, 0, 0, 1, 2, 0, 0, 24, 96, 0, 0, 10000, 60736, 0, 0, 20511168, 168661760, 0, 0, 134002359296, 1398597049856, 0, 0, 2146989255011328, 27232259080056832, 0, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,5
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COMMENTS
| Reference needed! [ N. J. A. Sloane (njas(AT)research.att.com) ]
a(n)=0 whenever A000217(n) is odd, because sum of atoms must be twice the sum of the bonds and both are integers. Full explanation on Munafo web page. [From Robert Munafo (mrob27(AT)gmail.com), Jun 22 2009]
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LINKS
| Robert Munafo, A Restricted Class of Permutations [From Robert Munafo (mrob27(AT)gmail.com), Jun 22 2009]
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EXAMPLE
| For n=4 the a(4)=2 solutions are 1-2-4*3 and 1-3=4=2 (where * is a triple bond) [From Robert Munafo (mrob27(AT)gmail.com), Jun 22 2009]
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CROSSREFS
| Sequence in context: A120582 A003784 A066294 * A179072 A073111 A013369
Adjacent sequences: A020913 A020914 A020915 * A020917 A020918 A020919
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KEYWORD
| nonn
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AUTHOR
| Mario Velucchi (mathchess(AT)velucchi.it)
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EXTENSIONS
| a(15) - a(18) from Robert Munafo (mrob27(AT)gmail.com), Jun 22 2009
a(19) - a(22) from Robert Munafo (mrob27(AT)gmail.com), Oct 02 2009
a(23) - a(26) from Robert Munafo (mrob27(AT)gmail.com), Oct 26 2009
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