Wednesday, 13 January 2016

calculus - Identity concerning complete elliptic integrals

It can be easily checked that both the complete elliptic integrals K(k),K(k) satisfy the same second order differential equation kk2d2ydk2+(13k2)dydkky=0 and hence from the theory of second order differential equations there is a relation of the form K(k)=cK(k)logk+f(k) where c is some constant and f(k) is some analytic function of k. The exact relation between K(k) and K(k) is given by K(k)=2K(k)πlog(4k)2[(12)2(112)k2+(1324)2(112+134)k4+] It can be verified with some patience that the RHS does satisfy the differential equation given above and thereby the relation between K(k) and K(k) can be established.



However is there an alternative proof based on the definition of K(k) and K(k) as complete elliptic integrals or using the hypergeometric relation 2K(k)π=2F1(12,12;1;k2) which can be presented to someone unaware of the theory of second order differential equations?

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