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A292186 Number of rooted unlabeled connected four-regular maps on a compact closed oriented surface with n vertices (and thus 2*n edges). 4
1, 3, 24, 297, 4896, 100278, 2450304, 69533397, 2247492096, 81528066378, 3280382613504, 145009234904922, 6986546222800896, 364418301804218028, 20459842995693256704, 1230262900677124568397, 78884016707711348637696, 5372823210133041283250178, 387394283866652086938107904 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
Equivalently, the number of rooted quadrangulations of oriented surfaces with n quadrangles (and thus 2*n edges).
Equivalently, the number of pairs of permutations (alpha,sigma) up to simultaneous conjugacy on a pointed set of size 4*n with alpha^2=sigma^4=1, acting transitively and without fixed points.
This is also the S(4, -6, 1) sequence of Martin and Kearney.
This sequence is not D-finite (or holonomic).
LINKS
L. Ciobanu and A. Kolpakov, Free subgroups of free products and combinatorial hypermaps, arXiv:1708.03842 [math.CO], 2017-2019.
R. J. Martin and M. J. Kearney, An exactly solvable self-convolutive recurrence, arXiv:1103.4936 [math.CO], 2011.
A. N. Stokes, Continued fraction solutions of the Riccati equation, Bull. Austral. Math. Soc. Vol. 25 (1982), 207-214.
FORMULA
a(0)=1, a(1)=3, a(n) = 4*n*a(n-1) + Sum_{k=1..n-2} a(k)*a(n-k-1) for n>=2.
From Peter Bala, Aug 22 2023: (Start)
The o.g.f. A(x) = 1 + 3*x + 24*x^2 + 297*x^3 + 4896*x^4 + 100278*x^5 + 2450304*x^6 + ... satisfies the Riccati differential equation (4*x^2)*A'(x) = -1 + (1 - 2*x)*A(x) - x*A(x)^2 with A(0) = 1.
O.g.f. as a continued fraction of Stieltjes type: 1/(1 - 3*x/(1 - 5*x/(1 - 7*x/(1 - 9*x/(1 - ... - (2*n+1)*x/(1 - ... )))))).
Also A(x) = 1/(1 + 2*x - 5*x/(1 - 3*x/(1 - 9*x/(1 - 7*x/(1 - ... - (4*n+1)*x/(1 - (4*n-1)*x/(1 - ... ))))))). (End)
PROG
(Python)
from sympy.core.cache import cacheit
@cacheit
def a(n): return n*2 + 1 if n < 2 else 4*n*a(n - 1) + sum([a(k)*a(n - k - 1) for k in range(1, n - 1)])
[a(n) for n in range(21)]
CROSSREFS
Sequence in context: A337027 A326001 A365998 * A365567 A258301 A365584
KEYWORD
nonn,easy
AUTHOR
Sasha Kolpakov, Sep 11 2017
STATUS
approved

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Last modified April 14 14:52 EDT 2024. Contains 371665 sequences. (Running on oeis4.)