A367118 - OEIS

A367118

Place n points in general position on each side of an equilateral triangle, and join every pair of the 3*n+3 boundary points by a chord; sequence gives number of regions in the resulting planar graph.

1, 13, 82, 307, 841, 1891, 3718, 6637, 11017, 17281, 25906, 37423, 52417, 71527, 95446, 124921, 160753, 203797, 254962, 315211, 385561, 467083, 560902, 668197, 790201, 928201, 1083538, 1257607, 1451857, 1667791, 1906966, 2170993, 2461537, 2780317, 3129106, 3509731, 3924073, 4374067

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OFFSET

0,2

COMMENTS

"In general position" implies that the internal lines (or chords) only have simple intersections. There is no interior point where three or more chords meet.

LINKS

Table of n, a(n) for n=0..37.

Scott R. Shannon, Image for n = 1.

Scott R. Shannon, Image for n = 2.

Scott R. Shannon, Image for n = 5.

FORMULA

Conjecture: a(n) = (1/4)*(9*n^4 + 12*n^3 + 15*n^2 + 12*n + 4).

a(n) = A367119(n) - A367117(n) + 1 by Euler's formula.

CROSSREFS

Cf. A367117 (vertices), A367119 (edges), A091908, A092098, A331782, A367015.

If the boundary points are equally spaced, we get A274585, A092866, A274586, A092867. - N. J. A. Sloane, Nov 09 2023

Sequence in context: A052255 A362545 A082203 * A101102 A213572 A380583

Adjacent sequences: A367115 A367116 A367117 * A367119 A367120 A367121

KEYWORD

nonn

AUTHOR

Scott R. Shannon and N. J. A. Sloane, Nov 05 2023

STATUS

approved