NIntegrate can do a number of different types of symbolic preprocessing on the integrand before starting the numerical calculations, including changes of variables.
Is there a way to find out what sort of symbolic preprocessing is being done and what is the precise integrand
NIntegrate is actually evaluating? For example, is it possible to find out if it does any changes of variables, and if yes, exactly what changes it does?
Motivation: I'm trying to understand what goes wrong here: Why do I get a different value when I change the order of integration? The question describes a simple oscillatory numerical integral in 2D.
NIntegrate gives different results depending on the order of integration variables, and no warnings. One result is wrong. Looking at the points sampled by
NIntegrate it's clear that Mathematica does a different change of variables depending on the order of integration variables.