In the pseudodifferential operator theory, we can define a pseudodifferential operator by $$a(x,D)u=(2\pi)^{n}\int{a(x,\xi)e^{i\langle xy,\xi \rangle}u(y)dyd\xi}$$ with $a(x,\xi)$ belong to some particular function space (denoted by $S^m$).In the Weyl calculus one adopts the symmetric compromise $$a^{w}(x,D)u=(2\pi)^{n}\int{a((\frac{x+y}{2}),\xi)e^{i\langle xy,\xi \rangle}u(y)dyd\xi}$$ again defined in the weak sense. From this one can see that the adjoint of $a^w$ is equal to $\bar a^{w}$. In particular, $a^w$ is its own adjoint when a is real valued. Is this convenience making Weyl calculus more applicable for physics? In mathematics, are there other reasons to the motivation of Weyl calculus? Furthermore, Can anyone show some problems which are solved by using this tool?

It is true that the initial motivation for Hermann Weyl in 1926 was linked to quantum mechanics and his convention was indeed ensuring that realvalued Hamiltonians get quantized by (formally) selfadjoint operators. On the other hand, the symplectic invariance of the Weyl calculus was discovered much later by André Weil: for $\chi\in Sp(n)$ ($Sp(n)$ is the linear symplectic group), there exists $U\in Mp(n)$ ($Mp(n)$ is the metaplectic group) such that $$ (a\circ \chi)^w=U^* a^w U. $$ There are many generalizations of that formula where $\chi$ is a canonical transformation not necessarily linear and the equality is replaced by some asymptotic equivalence. This result is as important as the change of variable formula in integrals. 


In the Weyl calculus there is closer agreement between operator and symbol composition than in the standard KohnNirenberg calculus. For example, $(a^w)^2\equiv (a^2)^w$ holds modulo order zero if $a$ has order one and is realvalued. Similarly, $(f\circ a)^w$ approximates $f(a^w)$, which is defined by the functional calculus, to higher order than the corresponding construction using the standard calculus. This is related to the symplectic invariance properties of the Weyl calculus, more specifically to the appearance of the Poisson bracket on the subprincipal level of the composition formula. Note that the Poisson bracket $\{f\circ a,a\}$ vanishes for realvalued symbols $a$. 

