

A111713


Number of reduced tree pairs of ncarets.


1



0, 1, 2, 14, 108, 930, 8700, 86598, 904176, 9804516, 109624536, 1257136130, 14726063264, 175650153588, 2128038439176, 26133761328150, 324786698542440, 4079191750094776, 51716838331485472, 661227615895716180, 8518677674587163584
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OFFSET

0,3


COMMENTS

The number of ordered pairs of rooted binary trees such that each tree has n carets and the pair is reduced. A caret is a vertex with two (downward) edges. Number the leaves of each tree from left to right (infix order). A treepair is reduced if i, i+1 is not the label of a caret in both trees for any i.
The elements of Thompson's group F can be represented uniquely as a reduced tree pair. a(n) is asymptotic to ((12/Pi)/mu) * mu^n/n^3*(1 + O(1/n)) and so the corresponding g.f. cannot be algebraic.


LINKS

Table of n, a(n) for n=0..20.
S. Cleary, M. Elder, A. Rechnitzer and J. Taback, Random subgroups of Thompson's group F, arxiv:0711.1343 (2007)
S. Cleary, M. Elder, A. Rechnitzer, J. Taback, Random subgroups of Thompson's group F, Groups, Geom. Dynam. 4 (1) (2010) 91126
Benjamin M. Woodruff, Statistical Properties of Thompson's Group and Random Pseudo Manifolds
Wikipedia, Thompson groups


FORMULA

a(n) = Sum_{k=1..n} (1)^(k+n) * binomial(k+1,nk) * ( binomial(2*k,k)/(k+1) )^2.
0 = (16*q^36*q^26*q+1)*A(q) + q*(4*q3)*(8*q^318*q^2+12*q1)*(d/dq)A(q) + q^2*(1+q)*(2*q1)*(16*q^216*q+1)*(d^2/dq^2)A(q)  4*q*(1+q)*(2*q1)^3.


CROSSREFS

Sequence in context: A108436 A088754 A103945 * A144278 A359108 A214766
Adjacent sequences: A111710 A111711 A111712 * A111714 A111715 A111716


KEYWORD

nonn


AUTHOR

Murray Elder, May 04 2007


STATUS

approved



