What is fubinis?

Fubini's is a mathematical theorem named after Italian mathematician Guido Fubini. The theorem relates to integration of multi-variable functions. The theorem states that if you are integrating a function with multiple variables, then the order of integration does not matter as long as the limits of integration are suitable for each variable. In other words, if you integrate a function in one variable first and then integrate the result in another variable, you will get the same answer as if you had integrated it in the reverse order. This theorem is widely used in calculus, physics, and engineering, among other fields. It has helped to simplify the mathematical calculations required for solving complex problems.