# Rigorous proof of Bohr-Sommerfeld quantization

Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In particular, I suppose it gives an exact description of the large quantum number asymptotics, which should be a theorem.

Also, is there a way to make the recipe more precise by adding corrections of some sort?

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You know about the WKB approximation, right? –  Aaron Nov 19 '11 at 21:01
Yes, it is called WKB and it has its own expansion series (I heard - a rather converging one). –  Vladimir Kalitvianski Nov 19 '11 at 21:12

Yes, it can be made precise and corresponds to the leading order of the semiclassical expansion (WKB approximation) in $\hbar$. See Faddeev-Yakubovsky's "Lectures on quantum mechanics for mathematics students" (§20, formula (13)). An approach inspired by geometric quantization is explained in chapter 4 in Bates-Weinstein's Lectures on the Geometry of Quantization.

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This answer addresses the geometrical origin of the Bohr-Sommerfeld condition. In geometric quantization, the additional structure required beyond the symplectic data of the phase space is a polarization. The quantization spaces are constructed as spaces of polarized sections with respect a polarization. The most "obvious" type of a polarization is the Kahler polarization, where the quantization spaces are spaces of holomorphic sections of a pre-quantum line bundle. Simple examples of systems which can be quantized by means of a Kahler polarization are the Harmonic oscillator and the spin. Another type of polarization is the real polarization (Please see for example the Blau's lecttures), which is locally equivalent to the polarization of a cotangent bundle. A real polarizations foilates the phase space (symplectic manifold) into Lagrangian submanifolds. When the leaves are compact, the quantum Hilbert space consists of sections with support only on certain leaves, which are exactly those which satisfy the Bohr-Sommerfeld condition. In this case, the quantum phase space is generated by distributional sections supported solely on the Bohr-Sommerfeld leaves (This result is due to Snyatycki). For example in the case of the spin, the Bohr-Sommerfeld leaves are small circles at half integer values of the $z$ coordinate in the two dimensional sphere. A more sophisticated example of Bohr-Sommerfeld leaves is the Gelfand-Cetlin system on flag manifolds.