A discussion of boundary conditions of ordinary differential equations, which arise in a variety of physical problems. Of the various methods of establishing the nonnegativity of Green's function, the one described here clarifies the role played by the characteristic values of the associated Sturm-Liouville problem.
Bellman, Richard Ernest, On the Nonnegativity of Green's Functions. Santa Monica, CA: RAND Corporation, 1963. https://www.rand.org/pubs/research_memoranda/RM3494.html. Also available in print form.
Bellman, Richard Ernest, On the Nonnegativity of Green's Functions, Santa Monica, Calif.: RAND Corporation, RM-3494-PR, 1963. As of October 06, 2021: https://www.rand.org/pubs/research_memoranda/RM3494.html