Green's Functions - http://math.arizona.edu/~lega/583/Fall98/lectnotes/GF1.html Lecture notes introducing this technique for solving first and second order differential equations. Scanned (GIF) by Joceline Lega.
Differential Equations with Boundary Value Problems - http://www.math.armstrong.edu/faculty/hollis/dewbvp By S. Hollis (Prentice Hall, 2002). Overview of contents, errata, software and link to purchasing page.
Waves and Oscillations - http://www.maths.gla.ac.uk/~fhg/waves/ Teaching resources developed by Frances Goldman, University of Glasgow.
Pseudo-Differential Operators - http://arxiv.org/abs/math.AP/9906155 These lecture notes cover a first year graduate course that was given on pseudo-differential operators. The calculus on manifolds is developed and applied to prove propagation of singularities and the Hodge decomposition theorem.
Vibrating Drumhead - http://www.ma.iup.edu/projects/CalcDEMma/drum/drum.html A set of Mathematica packages which outline solutions of the wave equation in various coordinates.
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