books.google.co.in - The central theme of this book is the search for solutions to eigenvalue problems that satisfy additional equations in the spectral parameter, for example, pairs of eigenvalue equations. This problem, which looks very simple at first, has turned out to be both deep and difficult. Moreover, this concept...http://books.google.co.in/books/about/The_bispectral_problem.html?id=J1EFmUAnFTYC&utm_source=gb-gplus-shareThe bispectral problem
