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A057655
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The circle problem: number of points (x,y) in square lattice with x^2 + y^2 <= n.
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27
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1, 5, 9, 9, 13, 21, 21, 21, 25, 29, 37, 37, 37, 45, 45, 45, 49, 57, 61, 61, 69, 69, 69, 69, 69, 81, 89, 89, 89, 97, 97, 97, 101, 101, 109, 109, 113, 121, 121, 121, 129, 137, 137, 137, 137, 145, 145, 145, 145, 149, 161, 161, 169, 177, 177, 177
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OFFSET
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0,2
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REFERENCES
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C. Alsina and R. B. Nelsen, Charming Proofs: A Journey Into Elegant Mathematics, Math. Assoc. Amer., 2010, p. 42.
J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 106.
F. Fricker, Einfuehrung in die Gitterpunktlehre, Birkhäuser, Boston, 1982.
P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 5.
E. Kraetzel, Lattice Points, Kluwer, Dordrecht, 1988.
C. D. Olds, A. Lax and G. P. Davidoff, The Geometry of Numbers, Math. Assoc. Amer., 2000, p. 51.
W. Sierpiński, Elementary Theory of Numbers, Elsevier, North-Holland, 1988.
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LINKS
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FORMULA
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a(n) = 1 + 4*{ [n/1] - [n/3] + [n/5] - [n/7] + ... }. - Gauss
a(n) = 1 + 4*Sum_{ k = 0 .. [sqrt(n)] } [ sqrt(n-k^2) ]. - Liouville (?)
a(n) - Pi*n = O(sqrt(n)) (Gauss). a(n) - Pi*n = O(n^c), c = 23/73 + epsilon ~ 0.3151 (Huxley). If a(n) - Pi*n = O(n^c) then c > 1/4 (Landau, Hardy). It is conjectured that a(n) - Pi*n = O(n^(1/4 + epsilon)) for all epsilon > 0.
a(n) = 1 + sum((floor(1/(k+1)) + 4 * floor(cos(Pi * sqrt(k))^2) - 4 * floor(cos(Pi * sqrt(k/2))^2) + 8 * sum((floor(cos(Pi * sqrt(i))^2) * floor(cos(Pi * sqrt(k-i))^2)), i = 1..floor(k/2))), k = 1..n). - Wesley Ivan Hurt, Jan 10 2013
G.f.: theta_3(0,x)^2/(1-x) where theta_3 is a Jacobi theta function. - Robert Israel, Sep 29 2014
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EXAMPLE
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a(0) = 1 (counting origin).
a(1) = 5 since 4 points lie on the circle of radius sqrt(1) + origin.
a(2) = 9 since 4 lattice points lie on the circle w/radius = sqrt(2) (along diagonals) + 4 points inside the circle + origin. - Wesley Ivan Hurt, Jan 10 2013
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MAPLE
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N:= 1000: # to get a(0) to a(N)
R:= Array(0..N):
for a from 0 to floor(sqrt(N)) do
for b from 0 to floor(sqrt(N-a^2)) do
r:= a^2 + b^2;
R[r]:= R[r] + (2 - charfcn[0](a))*(2 - charfcn[0](b));
od
od:
convert(map(round, Statistics:-CumulativeSum(R)), list); # Robert Israel, Sep 29 2014
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MATHEMATICA
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f[n_] := 1 + 4Sum[ Floor@ Sqrt[n - k^2], {k, 0, Sqrt[n]}]; Table[ f[n], {n, 0, 60}] (* Robert G. Wilson v, Jun 16 2006 *)
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PROG
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(PARI) a(n)=sum(x=-n, n, sum(y=-n, n, if((sign(x^2+y^2-n)+1)*sign(x^2+y^2-n), 0, 1)))
(PARI) a(n)=1+4*sum(k=0, sqrtint(n), sqrtint(n-k^2) ); /* Benoit Cloitre, Oct 08 2012 */
(Haskell)
a057655 n = length [(x, y) | x <- [-n..n], y <- [-n..n], x^2 + y^2 <= n]
(Python)
from math import isqrt
def A057655(n): return 1+(sum(isqrt(n-k**2) for k in range(isqrt(n)+1))<<2) # Chai Wah Wu, Jul 31 2023
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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STATUS
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approved
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