Real Vector Spaces and the Cauchy-Schwarz Inequality in ACL2(r)
This work provides a foundational formal verification for a key mathematical theorem, which is incremental as it extends existing theorem-proving capabilities to a new context.
The authors tackled the problem of formally proving the Cauchy-Schwarz inequality by developing a mechanical proof in ACL2(r), including formalizing real vector spaces as inner product spaces, and demonstrated its application by establishing continuity for functions in metric spaces.
We present a mechanical proof of the Cauchy-Schwarz inequality in ACL2(r) and a formalisation of the necessary mathematics to undertake such a proof. This includes the formalisation of $\mathbb{R}^n$ as an inner product space. We also provide an application of Cauchy-Schwarz by formalising $\mathbb R^n$ as a metric space and exhibiting continuity for some simple functions $\mathbb R^n\to\mathbb R$. The Cauchy-Schwarz inequality relates the magnitude of a vector to its projection (or inner product) with another: \[|\langle u,v\rangle| \leq \|u\| \|v\|\] with equality iff the vectors are linearly dependent. It finds frequent use in many branches of mathematics including linear algebra, real analysis, functional analysis, probability, etc. Indeed, the inequality is considered to be among "The Hundred Greatest Theorems" and is listed in the "Formalizing 100 Theorems" project. To the best of our knowledge, our formalisation is the first published proof using ACL2(r) or any other first-order theorem prover.