Comcepts:
http://en.wikipedia.org/wiki/Harmonic_function
http://en.wikipedia.org/wiki/Differentiable_manifold
http://en.wikipedia.org/wiki/Urysohn_lemma#Formal_statement
http://en.wikipedia.org/wiki/Densely_defined_operator
http://en.wikipedia.org/wiki/Partial_isometry
http://en.wikipedia.org/wiki/Gronwall’s_inequality