What is the final expression for the energy in separation of variables?

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SUMMARY

The final expression for energy in the context of separation of variables in partial differential equations (PDEs) is derived from the Schrödinger Equation (SE) for systems with two degrees of freedom. When applying separation of variables, the Hamiltonian operator acts on each component independently, leading to the equation $$\frac{G(x)}{g(x)} + \frac{H(y)}{h(y)} = E$$. This indicates that the energy E is the sum of two constants, each associated with the respective variables x and y. The choice of which equation to assign the energy does not affect the final expression, as E remains constant regardless of the separation method used.

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Suppose you have some partiel DE describing a physical system with 2 degrees of freedom (e.g. the SE). If you try separation of variables you get something like:

Hg(x)h(y) = Eg(x)h(y)

now you can separate this to two equations, but the energy has to go in one of them. Is the final expression for the energy dependent on which one you choose to put it in?
 
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E typically doesn't go with one or the other. When separation works, what happens is you get
$$\hat{H}[g(x)h(y)] = G(x)h(y) + g(x)H(y).$$ The Hamiltonian acts on each piece separately. Then you can divide both sides by g(x)h(y) to get
$$\frac{G(x)}{g(x)} + \frac{H(y)}{h(y)} = E.$$ The only way this can be satisfied for all x and y is if the two terms on the left are each constants. The energy is the sum of those constants.
 

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