OFFSET
0,4
COMMENTS
A PQ-tree is a rooted tree with P-type internal nodes that have at least 3 children that are reversibly ordered (the reverse of the order is equivalent to the order) and Q-type internal nodes that have at least 2 unordered children.
REFERENCES
F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Cambridge, 1998, p. 242 (3.3.91).
LINKS
Christian G. Bower, Table of n, a(n) for n = 0..127
FORMULA
E.g.f. satisfies A(x) = x + 1/(2-2A(x)) + exp(A(x)) - A(x)^2/2 - 3/2*(A(x)+1).
a(n) ~ 2^n * n^(n-1) * sqrt((2 - 10*s + 15*s^2 - 8*s^3 + s^5)/(-4 + 6*s - 3*s^2 - 3*s^3 + 2*s^4)) * ((s-1)^2/(-2 + 8*s - 7*s^2 + s^3 + s^4))^n / exp(n), where s = 0.4037320373976420090487567872... is the root of the equation exp(s) + 1/(2*(s-1)^2) = 5/2 + s. - Vaclav Kotesovec, Jan 08 2014
MATHEMATICA
CoefficientList[InverseSeries[Series[-E^x + (-2+x*(-2+x*(4+x)))/(2*(-1+x)), {x, 0, 20}], x], x] * Range[0, 20]! (* Vaclav Kotesovec, Jan 08 2014 *)
PROG
(PARI) read("transforms_pari.txt"); {pql(A) = A = trv_chain_l(A)+trv_exp(A)-opv_mul_egf(A, A)/2-2*A; A[1]=0; A} {apql(n) = local(SX, SY); SY = SX = [0, 1]; for(i=1, n, SY=concat(SY, 0); SX=concat(SX, 0); SY=SX+pql(SY)); SY} A136629(n) = apql(min(1, n-1))[n+1]
CROSSREFS
KEYWORD
nonn
AUTHOR
Christian G. Bower, Jan 14 2008
STATUS
approved