To solve this equation using the semiclassical approximation, each function must be expanded as a power series in ℏ ℏ . From the equations, the power series must start with at least an order of ℏ ℏ − − 1 to satisfy the real part of the equation; for a good classical limit starting with the highest power of the Planck constant possible is preferable, which leads to A ( x ) = 1 ℏ ℏ ∑ ∑ k = 0 ∞ ∞ ℏ ℏ k A k ( x ) and B ( x ) = 1 ℏ ℏ ∑ ∑ k = 0 ∞ ∞ ℏ ℏ k B k ( x ) , with the following constraints on the lowest order terms, A 0 ( x ) 2 − − B 0 ( x ) 2 = 2 m ( V ( x ) − − E ) and A 0 ( x ) B 0 ( x ) = 0.