

A343755


Number of regions formed by infinite lines when connecting all vertices and all points that divide the sides of an equilateral triangle into n equal parts.


3



7, 30, 144, 474, 1324, 2934, 5797, 10614, 17424, 27480, 41845, 61602, 85711, 120120, 159213, 207798, 269668, 349272, 434878, 545496, 661764, 804582, 973471, 1174980, 1374646, 1631304, 1908768, 2218254, 2560198, 2976486, 3378985, 3887796, 4405671, 4995240, 5617689, 6322878
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OFFSET

1,1


COMMENTS

The count of regions includes both the closed bounded polygons and the open unbounded areas surrounding these polygons with two edges that go to infinity. The number of unbounded areas appears to equal 6*(n^2  n + 1).
See A344279 for further examples and images of the regions.


LINKS

Table of n, a(n) for n=1..36.
Scott R. Shannon, Image for n = 1. In this and other images the triangle's vertices are highlighted as white dots while the outer open regions are crosshatched. The key for the edgenumber coloring is shown at the topleft of the image. Note the edge count for open areas also includes the two infinite edges
Scott R. Shannon, Image for n = 2.
Scott R. Shannon, Image for n = 3.
Scott R. Shannon, Image for n = 4.
Scott R. Shannon, Image for n = 5.
Scott R. Shannon, Image for n = 6.


FORMULA

Conjectured formula: a(n) = A344279(n) + 6*(n^2  n + 1).
Conjectured formula: a(n) = A344279(n) + A121205(n1), for n>=7.


EXAMPLE

a(1) = 7 as the three connected vertices of a triangle form one polygon along with six outer unbounded areas, seven regions in total.
a(2) = 30 as when the three vertices and three edges points are connected they form twelve polygons, all inside the triangle, along with eighteen outer unbounded areas, thirty regions in total.
a(2) = 144 as when the three vertices and six edges points are connected they form one hundred two polygons, seventyfive inside the triangle and twentyseven outside, along with fortytwo outer unbounded areas, one hundred fortyfour regions in total.


CROSSREFS

Cf. A344279 (number of polygons), A344657 (number of vertices), A344896 (number of edges), A346446 (number of kgons), A092867 (number polygons inside the triangle), A121205, A345025.
Sequence in context: A256981 A323929 A180786 * A026653 A271554 A296013
Adjacent sequences: A343752 A343753 A343754 * A343756 A343757 A343758


KEYWORD

nonn


AUTHOR

Scott R. Shannon, Jun 28 2021


STATUS

approved



