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In the context of synthetic differential geometry, we study the Laplace operator an a Riemannian manifold. The main new aspect is a neighbourhood of the diagonal, smaller than the second neighbourhood usually required as support for second order differential operators. The new neighbourhood has the property that a function is affine on it if and only if it is harmonic.
@article{TAC_2001_9_a0, author = {Anders Kock}, title = {Infinitesimal aspects of the {Laplace} operator}, journal = {Theory and applications of categories}, pages = {1--16}, publisher = {mathdoc}, volume = {9}, year = {2001}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2001_9_a0/} }
Anders Kock. Infinitesimal aspects of the Laplace operator. Theory and applications of categories, CT2000, Tome 9 (2001), pp. 1-16. http://geodesic.mathdoc.fr/item/TAC_2001_9_a0/