Eigenvectors and values

Finally digging into what will hopefully become a decent study pattern this summer. Today's topic was Eigenvectors and Eigenvalues. As my text has a copyright from 36 years ago, the applications were predictably systems of Linear Equations, Quadratic Equations, and optimization problems. Of course, the text does touch on the result that any positive semi-definite matrix can be reduced to SDS^-1, but it's presented as a pretty theoretical result. Who knew in 1980 that one of the world's largest companies (Google) would be built on that result?