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A128650 Number of polygons on n vertices with exactly three faces. 4
0, 4, 16, 60, 224, 840, 3168, 12012, 45760, 175032, 671840, 2586584, 9984576, 38630800, 149768640, 581690700, 2262890880, 8815845720, 34389986400, 134312002440, 525129633600, 2055166429680, 8050465600320, 31561484455800 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 2..200

T. Mansour and S. Severini, Grid polygons from permutations and their enumeration by the kernel method, arXiv:math/0603225 [math.CO], 2006.

FORMULA

a(n) = 4*(n-2)/(n-1) * C(2*n-4,n-2).

For n>2, a(n) = 4*A001791(n-2).

MATHEMATICA

Table[4 Binomial[2n-4, n-3], {n, 2, 25}] (* Jean-Fran├žois Alcover, Dec 05 2018 *)

CROSSREFS

Sequence in context: A269532 A269673 A231896 * A072335 A081161 A032106

Adjacent sequences:  A128647 A128648 A128649 * A128651 A128652 A128653

KEYWORD

nonn

AUTHOR

Ralf Stephan, May 08 2007

STATUS

approved

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Last modified January 25 01:33 EST 2020. Contains 331229 sequences. (Running on oeis4.)