

A020916


A molecule is a row of atoms joined together by bonds; each atom has a valence (e.g., 13=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
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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 124*3 and 13=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



