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 A020916 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) is the number of molecules with n atoms and different valencies from 1 to n. 1
 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; text; internal format)
 OFFSET 0,5 COMMENTS Reference needed! [ N. J. A. Sloane ] 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. - Robert Munafo, Jun 22 2009 LINKS Table of n, a(n) for n=0..26. Robert Munafo, A Restricted Class of Permutations [From Robert Munafo, Jun 22 2009] EXAMPLE For n=4 the a(4)=2 solutions are 1-2-4*3 and 1-3=4=2 (where * is a triple bond). - Robert Munafo, Jun 22 2009 CROSSREFS Sequence in context: A287982 A287506 A288125 * A179072 A073111 A229685 Adjacent sequences: A020913 A020914 A020915 * A020917 A020918 A020919 KEYWORD nonn,more AUTHOR Mario Velucchi (mathchess(AT)velucchi.it) EXTENSIONS a(15)-a(18) from Robert Munafo, Jun 22 2009 a(19)-a(22) from Robert Munafo, Oct 02 2009 a(23)-a(26) from Robert Munafo, Oct 26 2009 STATUS approved

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Last modified June 22 00:43 EDT 2024. Contains 373561 sequences. (Running on oeis4.)