What's the difference between double limit and repetition limit in postgraduate mathematics? What are the methods to find the double limit?

Double limit needs to be transformed into repeated limit in calculation;

Double limit in layman's terms, the integrals of x and y are mixed together;

Iterative restriction treats the two separately (divide and conquer), with Y before X or X before Y.

The difference mainly depends on the two sides of the integration area, and the former is selected parallel to the Y axis, otherwise.

Note also that the integral function is 1.

If the integration area is a circle, polar coordinates.