|
|
A006416
|
|
Number of loopless rooted planar maps with 3 faces and n vertices and no isthmuses. Also a(n)=T(4,n-3), array T as in A049600.
(Formerly M4490)
|
|
4
|
|
|
1, 8, 20, 38, 63, 96, 138, 190, 253, 328, 416, 518, 635, 768, 918, 1086, 1273, 1480, 1708, 1958, 2231, 2528, 2850, 3198, 3573, 3976, 4408, 4870, 5363, 5888, 6446, 7038, 7665, 8328, 9028, 9766, 10543, 11360, 12218, 13118, 14061, 15048
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
2,2
|
|
COMMENTS
|
If Y_i (i=1,2,3) are 2-blocks of an n-set X then, for n>=6, a(n-3) is the number of (n-3)-subsets of X intersecting each Y_i (i=1,2,3). - Milan Janjic, Nov 09 2007
a(n) is also the number of triangle subgraphs in a complete graph on n+3 vertices, minus 3 non-incident edges, for n > 2. - Robert H Cowen, Jun 23 2018
|
|
REFERENCES
|
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
|
|
LINKS
|
|
|
FORMULA
|
G.f.: x^2*(1+4*x-6*x^2+2*x^3)/(1-x)^4.
a(n-3) = (1/6)*n^3-(1/2)*n^2-(8/3)*n+6, n=6,7,... - Milan Janjic, Nov 09 2007
a(2)=1, a(3)=8, a(4)=20, a(5)=38, a(n)=4*a(n-1)-6*a(n-2)+4*a(n-3)-a(n-4). - Harvey P. Dale, Aug 25 2013
a(n+2) = Hyper2F1([-3, n], [1], -1). - Peter Luschny, Aug 02 2014
|
|
MAPLE
|
a := n -> hypergeom([-3, n-2], [1], -1);
seq(round(evalf(a(n), 32)), n=2..41); # Peter Luschny, Aug 02 2014
|
|
MATHEMATICA
|
CoefficientList[Series[(1+4x-6x^2+2x^3)/(1-x)^4, {x, 0, 50}], x] (* or *) LinearRecurrence[{4, -6, 4, -1}, {1, 8, 20, 38}, 50] (* Harvey P. Dale, Aug 25 2013 *)
f[n_]:= Binomial[n, 3] - 3(n-2); Table[{n, f[n]}, {n, 5, 100}]//TableForm (* Robert H Cowen, Jun 23 2018 *)
|
|
PROG
|
(PARI) Vec((1+4*x-6*x^2+2*x^3)/(1-x)^4 + O(x^40)) \\ Andrew Howroyd, Jul 15 2018
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,easy,nice
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|