Unlike most other elementary shapes, such as the circle and square, there is no algebraic equation to determine the perimeter of an ellipse. Throughout history, a large number of equations for approximations and estimates have been made for the perimeter of an ellipse.
An ellipse is defined by two axes: the major axis (the longest diameter) of length and the minor axis (the shortest diameter) of length , where the quantities and are the lengths of the semi-major and semi-minor axes respectively. The exact perimeter of an ellipse is given by the integral: [1]
where is the eccentricity of the ellipse, defined as [2]
If we define the function
known as the complete elliptic integral of the second kind, the perimeter can be expressed in terms of that function as simply
.
The integral used to find the area does not have a closed-form solution in terms of elementary functions.
Another solution for the perimeter, this time using the sum of a infinite series, is: [3]
where is the eccentricity of the ellipse.
More rapid convergence may be obtained by expanding in terms of Found by James Ivory, [4] Bessel [5] and Kummer, [6] there are several equivalent ways to write it. The most concise is in terms of the binomial coefficient with , but it may also be written in terns of the double factorial or integer binomial coefficients: The coefficients are slightly smaller (by a factor of ) than the preceding, but also is numerically much smaller than except at and . For eccentricities less than 0.5 (), the error is at the limits of double-precision floating-point after the term. [7]
Because the exact computation involves elliptic integrals, several approximations have been developed over time.
Indian mathematician Srinivasa Ramanujan proposed multiple approximations: [8] [9]
First approximation:
Second approximation:
where
Final approximation:
The final approximation in Ramanujan's notes on the perimeter of the ellipse is regarded as one of his most mysterious equations. It is
where
and is the eccentricity of the ellipse. [9]
Ramanujan did not provide any rationale for this formula.
This formula is simpler than most perimeter formulas but less accurate for highly eccentric ellipses.[ citation needed ]
In more recent years, computer programs have been used to find and calculate more precise approximations of the perimeter of an ellipse. In an online video about the perimeter of an ellipse, recreational mathematician and YouTuber Matt Parker, using a computer program, calculated numerous approximations for the perimeter of an ellipse. [10] Approximations Parker found include:
The authors recently found a power series for the perimeter of an ellipse whose variable is not eccentricity and which converges considerably faster than the standard series. Not finding it in the references available to us, we imagined it might be new. However, the referee informed us that we had rediscovered one of Kummer's quadratic transformations of Gauss's hypergeometric series, dating back to 1837.which cites to Kummer, Ernst Eduard (1836). "Uber die Hypergeometrische Reihe" [About the hypergeometric series]. Journal für die Reine und Angewandte Mathematik (in German). 15 (1, 2): 39–83, 127–172. doi:10.1515/crll.1836.15.39.