Rational approximations for values of the digamma function and a denominators conjecture

In 2007, A.I.Aptekarev and his collaborators discovered a sequence of rational approximations to Euler's constant $γ$ defined by a linear recurrence. In this paper, we generalize this result and present an explicit construction of rational approximations for the numbers $\ln(b)-ψ(a+1),$ $a, b\in {\mathbb Q},$ $b>0, a>-1,$ where $ψ$ defines the logarithmic derivative of the Euler gamma function. We prove exact formulas for denominators and numerators of the approximations in terms of hypergeometric sums. As a consequence, we get rational approximations for the numbers $π/2\pmγ.$ We compare the results obtained with those of T. Rivoal for the numbers $γ+\ln(b)$ and prove denominators conjectures proposed by Rivoal for denominators of rational approximations for $γ+\ln(b)$ and common denominators of simultaneous approximations for the numbers $γ$ and $ζ(2)-γ^2.$

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