Gauss circle problem
In mathematics, the Gauss circle problem is the problem of determining how many integer lattice points there are in a circle centred at the origin and with radius r. The first progress on a solution was made by Carl Friedrich Gauss, hence its name.
Consider a circle in R2 with centre at the origin and radius r ≥ 0. Gauss' circle problem asks how many points there are inside this circle of the form (m,n) where m and n are both integers. Since the equation of this circle is given in Cartesian coordinates by x2 + y2 = r2, the question is equivalently asking how many pairs of integers m and n there are such that
If the answer for a given r is denoted by N(r) then the following list shows the first few values of N(r) for r an integer between 0 and 10:
Bounds on a solution and conjecture
The area inside a circle of radius r is given by πr2, and since a square of area 1 in R2 contains one integer point, the expected answer to the problem could be about πr2. In fact it should be slightly higher than this, since circles are more efficient at enclosing space than squares. So in fact it should be expected that
for some error term E(r). Finding a correct upper bound for E(r) is thus the form the problem has taken.
Gauss managed to prove that
Writing |E(r)| ≤ Crt, the current bounds on t are
A much simpler sum appears if the sum of squares function r2(n) is defined as the number of ways of writing the number n as the sum of two squares. Then
Although the original problem asks for integer lattice points in a circle, there is no reason not to consider other shapes or conics, indeed Dirichlet's divisor problem is the equivalent problem where the circle is replaced by the rectangular hyperbola. Similarly one could extend the question from two dimensions to higher dimensions, and ask for integer points within a sphere or other objects. If one ignores the geometry and merely considers the problem an algebraic one of Diophantine inequalities then there one could increase the exponents appearing in the problem from squares to cubes, or higher.
The primitive circle problem
Another generalisation is to calculate the number of coprime integer solutions m, n to the equation
This problem is known as the primitive circle problem, as it involves searching for primitive solutions to the original circle problem. If the number of such solutions is denoted V(r) then the values of V(r) for r taking small integer values are
Using the same ideas as the usual Gauss circle problem and the fact that the probability that two integers are coprime is 6/π2, it is relatively straightforward to show that
As with the usual circle problem, the problematic part of the primitive circle problem is reducing the exponent in the error term. At present the best known exponent is 221/304 + ε if one assumes the Riemann hypothesis. Without assuming the Riemann hypothesis, the best known upper bound is
for a positive constant c. In particular, no bound on the error term of the form 1 − ε for any ε > 0 is currently known that does not assume the Riemann Hypothesis.
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- J. Wu, On the primitive circle problem, Monatsh. Math. 135 (2002), pp.69–81.