# Path integral in even number of spatial dimensions: does it exist?

The path integral formulation of Quantum Mechanics is related to Huygens principle, as stated by Feynman in his seminal article  and widely commented since then. However Huygens principle does not hold in even dimensions, see this web page for example, or this Physics SE. Can we conclude that path integral only work in odd dimensions then?

 Feynman, R. P. (Apr. 1948). “Space-Time Approach to Non-Relativistic Quantum Mechanics”. In: Rev. Mod. Phys. 20 (2), pp. 367–387

• There are other justifications for the path integral formulation of QM aside from Huygen's Principle, such as a least-action approach. The fact that Huygen's Principle doesn't apply doesn't mean that a path-integral formulation can't be done in 2-d, but it does impact what that path-integral formulation looks like. – Paul Jul 6 '18 at 13:35
• AFAIK, the path integral formulation is the lagrangian formulation of QM (instead of the Schrödinger's hamiltonian formulation) and has nothing to do with Huygens principle (even if this was an initial or historical justification). The path formulation should be valid for any dimensions. – Cham Jul 6 '18 at 15:44

He follows Dirac's epochal (1933) Physikalische Zeitschrift der Sowjetunion 3, 64–72 to actually demonstrate that if the amplitude of the wave is given on any "surface", its value at a short time after this is the sum of all contributions from all points of the surface at the original time, each contribution delayed in phase by an amount proportional to the action S of that segment, classically (extremally). This is the essence of QM amplitudes multiplied, concatenated, and summed over, $$\langle q_t| q_T\rangle =\int \langle q_t| q_m\rangle dq_m\langle q_m| q_{m-1}\rangle dq_{m-1} ... \langle q_2| q_1\rangle dq_1 \langle q_1| q_T\rangle,$$ eqn (11) of the above; and of Huygens' principle; today it is just about the second thing we all learn about QM, cf Dirac's QM book, §32. This is, in fact, up to normalization, the very definition of the path integral, and it works in any and all dimensions. This is what he calls the strong Huygens principle--definitely not what your links dictate.