Back in 1948, the journal Physical Review published a paper entitled “Space-Time Approach to Quantum Electrodynamics” by a young physicist named R.P. Feynman at Cornell University. The paper described ...
In this paper, we investigate under what circumstances the Laplace-Beltrami operator on a pseudo-Riemannian manifold can be written as a sum of squares of vector fields, as is naturally the case in ...
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