Some new results in non-commutative algebra in the calculation of a two-point function

Abstract
An explicit and finite algorithm is derived, allowing the exact expansion of z=exp(x+ lambda y) for non-commuting x and y, up to second order in lambda . Such an expansion is the inverse of the usual Hausdorff-Baker-Campbell formula. The matrix elements of z are computed, and as an application, the Duhamel two-point function obtained.
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