OFFSET
0,3
COMMENTS
Number of perfect matchings of an arc diagram with 2*n vertices, where neighboring vertices are joined by one arc, the vertices 1 and 2*n are not adjacent if n>=2, and all other pairs of vertices are joined by two arcs.
LINKS
Dmitry Efimov, The hafnian of Toeplitz matrices of a special type, perfect matchings and Bessel polynomials, arXiv:1904.08651 [math.CO], 2020.
FORMULA
a(n) = Sum_{k=0..n} (-1)^(n-k-1)*(n+k-1)!*(-3*n+k)/(k!*(n-k)!), n>=2.
D-finite with recurrence a(n+1) = (4n+4)*a(n) - (8n-13)*a(n-1) - 2*a(n-2), n>=4.
D-finite with recurrence a(n+1) = ((32*n^2-12*n+2)*a(n) + (8*n+1)*a(n-1))/(8*n-7), n>=3.
a(n) ~ (2*n)!/sqrt(e)*n!.
EXAMPLE
A symmetric 4 X 4 Toeplitz matrix A with the first row (0,1,2,0) has the form:
0 1 2 0
1 0 1 2
2 1 0 1
0 2 1 0.
Its hafnian equals Hf(A)= a12*a34 + a13*a24 + a14*a23 = 1*1 + 2*2 + 0*1 = 5.
MAPLE
[1, 1, seq(add((-1)^(n-k-1)*(n+k-1)!*(-3*n+k)/(k!*(n-k)!), k=0..n), n=2..32)] # Georg Fischer, Jun 05 2021
MATHEMATICA
Join[{1, 1}, RecurrenceTable[{a[n+1] == (4*n+4)*a[n]-(8*n-13)*a[n-1]-2*a[n-2], a[2]==5, a[3]==57, a[4]==859}, a[n], {n, 2, 32}]] (* Georg Fischer, Jun 05 2021 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Dmitry Efimov, Jul 16 2020
STATUS
approved