The equation you provided is a form of Fubini's Theorem or a related result in the context of change of variables in integrals, though it's not immediately a standard, named formula like some famous ones (e.g., substitution rule). It expresses a relationship between the integral of a function and the integral of its inverse.

To break it down:

$\int_{a}^{b} f(x) \, dx = \int_{a}^{b} f^{-1}(x) \, dy$

This equation essentially suggests that you can transform an integral with respect to $x$ into an integral with respect to $y$, where $y = f(x)$ and $f^{-1}(x)$ is the inverse of the function $f(x)$.

How this works:

1. The function $f$ is assumed to be bijective (one-to-one and onto) so that its inverse $f^{-1}$ exists.
2. When making the substitution $y = f(x)$, you use the fact that the bounds $a$ and $b$ in the original integral $\int_a^b f(x) \, dx$ are transformed under the inverse, and the differential $dx$ changes appropriately according to $dy = f'(x) \, dx$.
3. Essentially, the integral over $f(x)$ can be re-expressed as an integral over $f^{-1}(x)$, but this requires careful handling of the variable transformation.

Known Theorem:

This kind of result appears in integral calculus when you're using a variable change or substitution method, but it might not have a universally recognized, singular "name" beyond being a form of integral transformation, depending on the exact context you're working in (e.g., Riemann integration or Lebesgue integrals in advanced contexts).

Conclusion:

While the equation is not necessarily a well-known theorem with a specific name, it is based on the principles of changing variables in integrals and can be derived from the inverse function theorem or the substitution method.

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Would you like a more detailed breakdown of how this transformation works or an example to clarify the process?

Here are some related questions to explore:

1. What is the inverse function theorem, and how does it relate to integrals?
2. How do variable substitutions work in definite integrals?
3. Can the equation be generalized to integrals of higher dimensions (multivariable integrals)?
4. How do bijective functions influence the transformation of integrals?
5. How does the chain rule in calculus affect integral transformations?

Tip: When performing transformations in integrals, always remember to adjust both the limits of integration and the differential when switching variables.