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A187452 Number of right isosceles triangles that can be formed from the n^2 points of n X n grid of points (or geoboard). 15
0, 4, 28, 96, 244, 516, 968, 1664, 2680, 4100, 6020, 8544, 11788, 15876, 20944, 27136, 34608, 43524, 54060, 66400, 80740, 97284, 116248, 137856, 162344, 189956, 220948, 255584, 294140, 336900, 384160, 436224, 493408, 556036, 624444, 698976, 779988, 867844 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This counts triples of distinct points A,B,C such that A,B,C are the vertices of an isosceles triangle with nonzero area, where the angle at B is a right angle. The triangles can have any orientation.

LINKS

Nathaniel Johnston and Colin Barker, Table of n, a(n) for n = 1..1000 [first 73 terms from Nathaniel Johnston]

Margherita Barile, MathWorld -- Geoboard.

Jessica Gonzalez, Illustration of a(3)=28

Nathaniel Johnston, C program for computing terms

Index entries for linear recurrences with constant coefficients, signature (4,-5,0,5,-4,1).

FORMULA

Empirical: a(n)=4*a(n-1)-5*a(n-2)+5*a(n-4)-4*a(n-5)+a(n-6). [R. H. Hardin, Apr 30 2011]

Empirical g.f.: 4*x*(x^2+3*x+1)/((1+x)*(1-x)^5). - N. J. A. Sloane, Apr 12 2016

Both the recurrence and the g.f. are true. For proof see [Paper in preparation]. - Warren D. Smith, Apr 17 2016

From Colin Barker, Apr 25 2016: (Start)

a(n) = (3-3*(-1)^n-16*n^2+10*n^4)/24.

a(n) = (5*n^4-8*n^2)/12 for n even.

a(n) = (5*n^4-8*n^2+3)/12 for n odd.

(End)

EXAMPLE

For n=2 if the four points are labeled

ab

cd

then the triangles are abc, abd, acd, bcd,

so a(2)=4.

For n=3, label the points

abc

def

ghi

The triangles are: abd (4*4 ways), acg (4 ways), ace and dbf (4 ways each), for a total of a(3) = 28. - N. J. A. Sloane, Jun 30 2016

MAPLE

with(linalg):

IsTriangle:=proc(points) local a, b, c; a:=points[3]-points[2]: b:=points[3]-points[1]: c:=points[2]-points[1]: if evalf(norm(a, 2)+norm(b, 2))>evalf(norm(c, 2)) and evalf(norm(a, 2)+norm(c, 2))>evalf(norm(b, 2)) and evalf(norm(b, 2)+norm(c, 2))>evalf(norm(a, 2)) then true: else false: fi: end:

IsRectangularTriangle:=proc(points) local a, b, c; a:=points[3]-points[2]: b:=points[3]-points[1]: c:=points[2]-points[1]: if IsTriangle(points) then if dotprod(a, b)=0 or dotprod(a, c)=0 or dotprod(b, c)=0 then true: else false: fi: else false: fi; end:

IsIsoscelesTriangle:=proc(points) local a, b, c; a:=points[3]-points[2]: b:=points[3]-points[1]: c:=points[2]-points[1]: if IsTriangle(points) then if norm(a, 2)=norm(b, 2) or norm(a, 2)=norm(c, 2) or norm(b, 2)=norm(c, 2) then true: else false: fi: else false: fi; end:

IsRectangularIsoscelesTriangle:=proc(points) if IsRectangularTriangle(points) and IsIsoscelesTriangle(points) then true: else false: fi: end:

a:=proc(n) local P, TriangleSet, i, j, a, b, c; P:=[]: for i from 0 to n do for j from 0 to n do P:=[op(P), [i, j]]: od; od; TriangleSet:={}: for a from 1 to nops(P) do for b from a+1 to nops(P) do for c from b+1 to nops(P) do if IsRectangularIsoscelesTriangle([P[a], P[b], P[c]]) then TriangleSet:={op(TriangleSet), [P[a], P[b], P[c]]}; fi; od; od; od; return(nops(TriangleSet)): end:

MATHEMATICA

LinearRecurrence[{4, -5, 0, 5, -4, 1}, {0, 4, 28, 96, 244, 516}, 40] (* Harvey P. Dale, Apr 29 2016 *)

PROG

(PARI) concat(0, Vec(4*x^2*(1+3*x+x^2)/((1-x)^5*(1+x)) + O(x^50))) \\ Colin Barker, Apr 25 2016

CROSSREFS

Cf. A045996, A077435, A186434, A189416.

Sequence in context: A294315 A263239 A296015 * A173296 A077595 A201243

Adjacent sequences:  A187449 A187450 A187451 * A187453 A187454 A187455

KEYWORD

nonn,easy

AUTHOR

Martin Renner, Apr 10 2011, Apr 13 2011

EXTENSIONS

a(10) - a(36) from Nathaniel Johnston, Apr 25 2011

STATUS

approved

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Last modified December 14 12:28 EST 2018. Contains 318097 sequences. (Running on oeis4.)