[en] The problem is discussed of extracting the QCD quark and gluon condensates from hadronic low-energy experimental data. (Z.J.). 4 figs., 14 refs

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[en] We derive optimal bounds on the Isgur-Wise function, taking into account the information about the measured masses of the upsilon poles present in the heavy meson elastic form factor. In the large b-quark mass limit, when the poles move towards the B anti B threshold, our results approach the bounds originally derived by de Rafael and Taron. (orig.)

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[en] We derive a compact expression for the Borel sum of a QCD amplitude in terms of the inverse Mellin transform of the corresponding Borel function. The result allows us to investigate the momentum-plane analyticity properties of the Borel-summed Green functions in perturbative QCD. An interesting connection between the asymptotic behaviour of the Borel transform and the Landau singularities in the momentum plane is established. We consider for illustration the polarization function of massless quarks and the resummation of one-loop renormalon chains in the large-β₀ limit, but our conclusions have a more general validity. (author)

[en] The stable analytic extrapolation of the scattering amplitudes with positive imaginary parts from the whole boundary or from a part of it to interior points are investigated. By using the Lagrange technique in convex optimization, a system of singular integral equations is derived which completely solve the problem. In the case of the extrapolation from the whole boundary this system reduces to the Fredholm equation recently obtained by Auberson et al., using different considerations, in connection with a related extremum problem. The present results give also the correct analytic-extrapolation formulae in the limiting case of scattering data which are exactly the restriction to the boundary of some analytic function. (author)