A form of this operation named after Jean-Gaston Darboux (10[1])was extended to higher dimensions by Thomas Stieltjes (STEEL-jee-iss). (10[1])An inequality for switching this operation with the limit inferior was proven by Pierre Fatou. Linear combinations of indicator functions called simple functions are used to define a form of this operation for which the dominated and monotone convergence theorems hold. This operation is the same for two functions that differ only on a null set. One form of this operation is defined by using the Lebesgue (luh-BEG) measure. (10[1])In (10[2])some cases, Fubini's theorem allows for (10[1])two instances of this operation to be interchanged. A form of this operation is defined when upper and lower sums named for Bernhard (10[1])Riemann agree. For (10[1])10 points, name this operation from calculus that finds the area under a (10[1])curve. ■END■