OFFSET
1,7
COMMENTS
Lunnon's DR(n,n-1)-DE(n,n-1). Knuth describes methodology for a similar enumeration, that of free trees with n nodes.
REFERENCES
D. E. Knuth, Fundamental Algorithms, 3d Ed. 1997, pp. 386-88.
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 1..1000
W. F. Lunnon, Counting Multidimensional Polyominoes, Computer Journal, Vol. 18 (1975), pp. 366-67.
FORMULA
G.f.: C(x)-C^2(x)/2+C(-x^2)/2 where C(x) is g.f. for same sequence with one cell labeled, A045648.
a(n) ~ c * d^n / n^(5/2), where d = 2.58968405406171542574769690513208346256... and c = 0.36257350770010314582973624284... . - Vaclav Kotesovec, Feb 29 2016
MATHEMATICA
s[ n_, k_ ] := s[ n, k ]=c[ n+1-k ]+If[ n<2k, 0, s[ n-k, k ](-1)^k ]; c[ 1 ]=1; c[ n_ ] := c[ n ]=Sum[ c[ i ]s[ n-1, i ]i, {i, 1, n-1} ]/(n-1); Table[ c[ i ]-Sum[ c[ j ]c[ i-j ], {j, 1, i/2} ]+If[ OddQ[ i ], 0, c[ i/2 ](c[ i/2 ]+(-1)^(i/2))/2 ], {i, 1, 33} ]
CROSSREFS
KEYWORD
easy,nice,nonn
AUTHOR
STATUS
approved