http://reference.wolfram.com/mathematica/tutorial/DSolveSecondOrderPDEs.html

http://www.math.psu.edu/tseng/class/Math251/Notes-PDE%20pt1.pdf

http://www.maths.bath.ac.uk/~masrs/ma20010/oderevision.pdf

http://www.ucl.ac.uk/~ucahhwi/LTCC/sectionD-classification.pdf

http://www.math.psu.edu/tseng/class/Math251/Notes-PDE%20pt3.pdf

http://people.math.sfu.ca/~stockie/teaching/math3073/m3073a1soln.pdf

http://www.math.nps.navy.mil/~bneta/soln.pdf

http://www.maths.manchester.ac.uk/~cp/sep.pdf

http://www.math.psu.edu/tseng/class/Math251/Notes-PDE%20pt4.pdf

http://banach.millersville.edu/~bob/math467/Review/main.pdf

http://www.maths.manchester.ac.uk/~kd/ma2m1/pdelec.pdf