I've recently been reading up on the problem of finding a closed-form solution to the perimeter of an ellipse. I am aware that, in the case of an ellipse, the standard formula used to derive arc-length results in a non-elementary integral. Would anybody be able to point me in the direction of recent research that has been done on the problem, recent breakthroughs related to the problem, or areas of mathematics that would be useful to look into in order to understand the problem better? PDF links to research would be ideal.
Thank you!