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### Abstract

We formalize with Isabelle/HOL a proof of a theorem by J. Hancl asserting the
irrationality of the sum of a series consisting of rational numbers, built up
by sequences that fulfill certain properties. Even though the criterion is a
number theoretic result, the proof makes use only of analytical arguments. We
also formalize a corollary of the theorem for a specific series fulfilling the
assumptions of the theorem.