High Quality Content by WIKIPEDIA articles! In mathematics, especially in the area of algebra known as module theory, Schanuel's lemma allows one to compare how far modules depart from being projective. It is useful in defining the Heller operator in the stable category, and in giving elementary descriptions of dimension shifting. The above argument may also be generalized to long exact sequences. Stephen Schanuel discovered the argument in Irving Kaplansky's homological algebra course at the University of Chicago in Autumn of 1958.