An elementary and unified proof of Grothendieck’s inequality

Abstract

We present an elementary, self-contained proof of Grothendieck’s inequality that unifies the real and complex cases and yields both the Krivine and Haagerup bounds, the current best-known explicit bounds for the real and complex Grothendieck constants respectively. This article is intended to be pedagogical, combining and streamlining known ideas of Lindenstrauss–Pełczynski, Krivine, and Haagerup into a proof that needs only univariate calculus, basic complex variables, and a modicum of linear algebra as prerequisites.