A215763 Number of undirected labeled graphs on n nodes with exactly 3 cycle graphs as connected components.

1, 6, 25, 120, 742, 5446, 45559, 427275, 4437279, 50532669, 626068404, 8382651732, 120614527728, 1855938746160, 30411098170560, 528670269132480, 9718179216603840, 188342519176964160, 3838144417230700800, 82046606738914540800, 1835772107381126208000

Offset

3,2

Links

Alois P. Heinz, Table of n, a(n) for n = 3..175

Example

a(4) = 6:  .1.2o  .1-2.  o1.2.  o1.2o  o1.2.  .1.2o
.          .|...  .....  ...|.  .....  ../..  ..\..
.          .3.4o  o3.4o  o3.4.  .3-4.  .3.4o  o3.4.

Maple

T:= proc(n, k) option remember; `if`(k<0 or k>n, 0, `if`(n=0, 1,
      add(binomial(n-1, i)*T(n-1-i, k-1)*ceil(i!/2), i=0..n-k)))
    end:
a:= n-> T(n, 3):
seq (a(n), n=3..25);

Crossrefs

Column k=3 of A215771.

Sequence in context: A227914 A179603 A298700 * A153481 A099359 A073967

Adjacent sequences: A215760 A215761 A215762 * A215764 A215765 A215766

Keyword

nonn

Author

Alois P. Heinz, Aug 23 2012

Status

approved