A063689 Number of 2-trees rooted at a triangle with 3 similar edges.

1, 1, 2, 6, 21, 83, 356, 1599, 7434, 35381, 171508, 843419, 4197179, 21094355, 106915928, 545859112, 2804656069, 14491370996, 75248398034, 392476363133, 2055245992376, 10801442696736, 56953957110855, 301207378815752, 1597342159296786, 8492297139795170

Offset: 1

Author

Vladeta Jovovic, Aug 22 2001

Keywords

• nonn

Examples

			Sequence really begins 1, 0, 0, 1, 0, 0, 2, 0, 0, 6, 0, 0, 21, 0, 0, 83, 0, 0, 356, ... but only nonzero trisection is shown.
		

References

• F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 76, Eq. (3.5.17).

Crossrefs

Cf. A005750, A058870, A058866, A054581.

Formula

a(n) = A058870(n) + A063687(n) with A058870(0)=0. - Sean A. Irvine, May 07 2023

Extensions

More terms from Sean A. Irvine, May 07 2023