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count=0
for x in range (-n, 2*n):
for y in range (-2*n, 2*n):
if (x*x+y*y>n*n and y<-tan*x+2*tan*n and y>tan*x-2*tan*n and x>-n):
count=count+1
print(count)
. count=0
. for x in range (-n, 2*n):
.. for y in range (-2*n, 2*n):
... if (x*x+y*y>n*n and y<-tan*x+2*tan*n and y>tan*x-2*tan*n and x>-n):
.... count=count+1
. print(count)
a(n) is the number of lattice points in a Cartesian grid between an equilateral triangle and an inscribed circle of radius n; one of the side of triangle is perpendicular to the X-axis; the сirclecircle's center is at the origin.
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(PARI) a(n) = sum(x=-n+1, 2*n, sum(y=-2*n, 2*n, ((x^2+y^2) > n^2) && (3*y^2 < (x-2*n)^2))); \\ Michel Marcus, May 22 2018
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