I tried evaluating the following double integral on Wolfram Alpha but got nothing.
$$ \int_0^\infty \int_0^\infty \exp \left [ -(x_1^{a_1} + x_2^{a_2})^{a_3} \right ] \ dx_1\ dx_2 $$ Assume that $a_1, a_2, a_3 > 0$
How does one evaluate such an integral. Is it even possible?