Textbook in PDF format

Heun's equation is a second-order differential equation which crops up in a variety of forms in a wide range of problems in applied mathematics. These include integral equations of potential theory, wave propagation, electrostatic oscillation, and Schrodinger's equation. This volume brings together important research work for the first time, providing an important resource for all those interested in this mathematical topic. Both the current theory and the main areas of application are surveyed, and includes contributions from authoritative researchers.
Heun's equation
Confluent Heun equation
Double Confluent Heun equation
Biconfluent Heun equation
Triconfluent Heun equation
Addendum: Classification
Bibliography