Do calculations from inside to outside variables.

Integrate with respect to x1 first while fixing every other variables.

Then, do the same integration for y1, x2, and y2 in order.

I don't know why the square bracket is missing though.

Best regards,

Junsik Yoon

Hi, I just tried with MAPLE. The first integral w.r.t. x1 has a closed form but then, when integrating the result w.r.t. y1, MAPLE cannot find an analytical expression.

I think that a numerical evaluation of this integral is the only solution.