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assumed to be an infinite series of contributions:

:

Replacing in the previous expression, we obtain:

:

Now we identify y₀ with some explicit expression on the right, and y_i, i = 1, 2, 3, ..., with some expression on the right containing terms of lower order than i. For instance:

:

In this way, any contribution can be explicitly calculated at any order. If we settle for the four first terms, the approximant is the following:

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