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Domain of Elliptic Integral of first kind (EllipticK)

Posted 10 years ago

I'm having trouble with this elliptic integral of first kind, the one denoted in Mathematica as EllipticK. This function should be defined for every value in the complex plane, however in Mathematica its domain in only x<1. How can I get its values for other points?

Moreover what is quite strange is that even if the ratio EllipticK[1-x]/EllipticK[x] is defined only in the domain [0, 1], if I ask to plot the exponential of this function, that is Exp[EllipticK[1-x] / EllipticK[x]] the plot actually covers the whole domain x<1, even though Mathematica is unable to give me the domain of this exponential function.

Can you help me in understanding what's going on? Thank you!

POSTED BY: Paola Ruggiero
2 Replies

Somehow I can not see the problem:

Plot3D[Through[{Re, Im}@Exp[EllipticK[x + I y]]], {x, -3, 3}, {y, -3, 3}]

gives:

enter image description here

I would be very surprised if there were any function Mathematica could not evaluate for complex arguments (except for mathematical reasons).

Regards -- Henrik

POSTED BY: Henrik Schachner

It appears that Mathematica evaluates EllipticK[k] only for real k. However, the MathWorld article

http://mathworld.wolfram.com/CompleteEllipticIntegraloftheFirstKind.html

has some formulas that can be applied for complex k.

POSTED BY: S M Blinder
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