Louis de Branges & James Rovnyak 
Square Summable Power Series 

This text for advanced undergraduate and graduate students introduces Hilbert space and analytic function theory, which is centered around the invariant subspace concept. The book’s principal feature is the extensive use of formal power series methods to obtain and sometimes reformulate results of analytic function theory. The presentation is elementary in that it requires little previous knowledge of analysis, but it is designed to lead students to an advanced level of performance. This is achieved chiefly through the use of problems, many of which were proposed by former students. The book’s tried-and-true approach was developed from the authors’ lecture notes on courses taught at Lafayette College, Bryn Mawr College, and Purdue University.