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A337641 One-quarter of the number of regions in the central square of an equal-armed cross with arms of length n (as in A331456). 2
1, 14, 70, 231, 576, 1207, 2255, 3883, 6267, 9588, 14088, 20021, 27667, 37306, 49240, 63859, 81517, 102603, 127545, 156769, 190739, 229932, 274898, 326181, 384332, 449878, 523472, 605766, 697380, 799053, 911449, 1035371, 1171471, 1320566, 1483374, 1660873, 1853819, 2063133, 2289607, 2534326, 2798159 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Without loss of generality, we may assume the central square has vertices (0,0), (1,0), (0,1), (1,1).

Suppose n >= 1. Then all nodes in the graph, whether or not in the central square, have rational coordinates with denominator at most 4*n^2 + 2*n + 1, which for n = 1, 2, 3, ... is 7, 21, 43, 73, 111, ... (cf. A054569).

This maximum is always attained, for example by the node at the intersection of the lines 2*n*x + y = n, joining (0,n) to (1, -n) and -x + (2*n+1)*y = n, joining (-n,0) to (n+1,1).

In the central square, the maximum number of sides in any region is (for n = 0, 1, 2, 3, ...) 3, 4, 6, 6, 6, 6, 6, 6, 6, 7, 7, 6, 6, 6, 6, 6, 7, 6, 7, 7, 6, 6, 6, 6, 6, 6, 6, 7, 7, 6, 6, 6, 6, 6, 6, 6, 7, 7, 6, 6, 6, ...  We conjecture that 7 is the maximum. - Lars Blomberg, Sep 19 2020.

See A331456 for further illustrations.

LINKS

Lars Blomberg, Table of n, a(n) for n = 0..74

Scott R. Shannon, Colored illustration for a(0): there are 4 regions, so a(0) = 1.

Scott R. Shannon, Colored illustration for a(1): there are 56 regions, so a(1) = 14.

Scott R. Shannon, Colored illustration for a(2): there are 280 regions, so a(2) = 70.

Scott R. Shannon, Colored illustration for a(3): there are 924 regions, so a(3) = 231.

Scott R. Shannon, Black and white illustration for a(1) (Shows vertices and regions for each square)

Scott R. Shannon, Black and white illustration for a(2) (Shows vertices and regions for each square)

Scott R. Shannon, Black and white illustration for a(3) (Shows vertices and regions for each square)

Scott R. Shannon, Black and white illustration for a(4) (Shows vertices and regions for each square)

Scott R. Shannon, Black and white illustration for a(5) (Shows vertices and regions for each square)

Scott R. Shannon, Black and white illustration for a(6) (Shows vertices and regions for each square)

CROSSREFS

Cf. A054569, A331456, A337640.

Sequence in context: A212751 A212749 A201106 * A213160 A268399 A034554

Adjacent sequences:  A337638 A337639 A337640 * A337642 A337643 A337644

KEYWORD

nonn,more

AUTHOR

Lars Blomberg, Scott R. Shannon, and N. J. A. Sloane, Sep 17 2020

STATUS

approved

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Last modified May 13 07:08 EDT 2021. Contains 343836 sequences. (Running on oeis4.)