Elliptic K
From specialfunctionswiki
The Elliptic $K$ function is also known as the complete Elliptic integral of the first kind. If $m=k^2$ we define the complete elliptic integral of the first kind, $K$ to be $$K(k)=K(m)=\displaystyle\int_0^{\frac{\pi}{2}} \dfrac{1}{\sqrt{1-k^2\sin^2 \theta}} d\theta.$$ The incomplete elliptic integral of the first kind is $$K(\phi |k) = K(\phi |m) = \displaystyle\int_0^{\phi} \dfrac{1}{\sqrt{1-k^2\sin^2 \theta}} d\theta.$$
