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.

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

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