What's F(a|b) in the MuPAD ode::solve output?

I tried to solve an ode equation with the help of MuPAD. I wrote:

 assume(g<>0): assume(.1<phi0<PI/2-.1):
 ode::solve(ode({phi''(t)=g/l*cos(phi(t)),phi(0)=phi0,phi'(0)=sqrt(2*g*l*sin(phi0))},phi(t)))

The solution contained many sigma parameters and in someone among these I found a strange function F(a|b) where a and b were another parameters. How can I interpret this F function?

I care about to say I'm not quite a wizard with this kind of things.

[Torsten]

I guess F is the Jacobi amplitude function:

http://www.wolframalpha.com/input/?i=w%22%28x%29%2Ba*cos%28w%28x%29%29%3D0

Best wishes

Torsten.

Answer 1

[Eric Lin]

It appears to be the Incomplete elliptic integral of the first kind

[Gaetano Lazzara]

I didn't know this function. You learn something new everyday.

Thank you.