Lemniscatic elliptic function

In mathematics, a lemniscatic elliptic function is an elliptic function related to the arc length of a lemniscate of Bernoulli studied by Giulio Carlo de' Toschi di Fagnano in 1718. It has a square period lattice and is closely related to the Weierstrass elliptic function when the Weierstrass invariants satisfy g₂ = 1 and g₃ = 0.

In the lemniscatic case, the minimal half period ω₁ is real and equal to

${\frac {\Gamma ^{2}\left({\frac {1}{4}}\right)}{4{\sqrt {\pi }}}}$

where Γ is the gamma function. The second smallest half period is pure imaginary and equal to iω₁. In more algebraic terms, the period lattice is a real multiple of the Gaussian integers.

The constants e₁, e₂, and e₃ are given by

$e_{1}={\tfrac {1}{2}},\qquad e_{2}=0,\qquad e_{3}=-{\tfrac {1}{2}}.$

The case g₂ = a, g₃ = 0 may be handled by a scaling transformation. However, this may involve complex numbers. If it is desired to remain within real numbers, there are two cases to consider: a > 0 and a < 0. The period parallelogram is either a square or a rhombus.

Lemniscate sine and cosine functions

The lemniscate sine (Latin: sinus lemniscatus) and lemniscate cosine (Latin: cosinus lemniscatus) functions sinlemn aka sl and coslemn aka cl are analogues of the usual sine and cosine functions, with a circle replaced by a lemniscate. They are defined by

$\operatorname {sl} (r)=s$

where

$r=\int _{0}^{s}{\frac {dt}{\sqrt {1-t^{4}}}}$

and

$\operatorname {cl} (r)=c$

where

$r=\int _{c}^{1}{\frac {dt}{\sqrt {1-t^{4}}}}.$

They are doubly periodic (or elliptic) functions in the complex plane, with periods 2πG and 2πiG, where Gauss's constant G is given by

$G={\frac {2}{\pi }}\int _{0}^{1}{\frac {dt}{\sqrt {1-t^{4}}}}=0.8346\ldots .$

Arclength of lemniscate

A lemniscate of Bernoulli and its two foci

The lemniscate of Bernoulli

$\left(x^{2}+y^{2}\right)^{2}=x^{2}-y^{2}$

consists of the points such that the product of their distances from the two points (1/√2, 0), (−1/√2, 0) is the constant 1/2. The length r of the arc from the origin to a point at distance s from the origin is given by

$r=\int _{0}^{s}{\frac {dt}{\sqrt {1-t^{4}}}}.$

In other words, the sine lemniscatic function gives the distance from the origin as a function of the arc length from the origin. Similarly the cosine lemniscate function gives the distance from the origin as a function of the arc length from (1, 0).

See also

• Gauss's constant
• Equianharmonic case

References

• Abramowitz, Milton; Stegun, Irene Ann, eds. (1983) [June 1964]. "Chapter 18". Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. Applied Mathematics Series. 55 (Ninth reprint with additional corrections of tenth original printing with corrections (December 1972); first ed.). Washington D.C.; New York: United States Department of Commerce, National Bureau of Standards; Dover Publications. p. 658. ISBN 978-0-486-61272-0. LCCN 64-60036. MR 0167642. LCCN 65-12253.
• Reinhardt, W. P.; Walker, P. L. (2010), "Lemniscate lattice", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248
• Siegel, C. L. (1969). "Topics in complex function theory. Vol. I: Elliptic functions and uniformization theory". Interscience Tracts in Pure and Applied Mathematics. 25. New York-London-Sydney: Wiley-Interscience A Division of John Wiley & Sons. ISBN 0-471-60844-0. MR 0257326.

External links

• "Lemniscate functions", Encyclopedia of Mathematics, EMS Press, 2001 [1994]