Renormalon
In physics, a renormalon (a term suggested by 't Hooft) is a particular source of divergence seen in perturbative approximations to quantum field theories (QFT). When a formally divergent series in a QFT is summed using Borel summation, the associated Borel transform of the series can have singularities as a function of the complex transform parameter. The renormalon is a possible type of singularity arising in this complex "Borel plane", and is a counterpart of an instanton singularity. Associated with such singularities, renormalon contributions are discussed in the context of quantum chromodynamics (QCD) and usually have the power-like  form formula_1 as functions of the momentum formula_2 (here formula_3 is the momentum cut-off). They are cited against the usual logarithmic effects like formula_4. Brief history. Perturbation series in quantum field theory are usually divergent as was firstly indicated by Freeman Dyson. According to the Lipatov method, formula_5-th order contribution of perturbation theory into any quantity can be evaluated at large formula_5 in the saddle-point approximation for functional integrals and is determined by instanton configurations. This contribution behaves usually as formula_7 in dependence on formula_5 and is frequently associated with approximately the same (formula_7) number of Feynman  diagrams. Lautrup has noted that there exist individual diagrams giving approximately the same contribution. In principle, it is possible that such diagrams are automatically taken into account in Lipatov's calculation, because its interpretation in terms of diagrammatic technique is problematic. However, 't Hooft put forward a conjecture that Lipatov's and Lautrup's contributions are related with different types of singularities in the Borel plane, the former with instanton ones and the latter with renormalon ones. Existence of instanton singularities is beyond any doubt, while existence of renormalon ones was never proved rigorously in spite of numerous efforts. Among  the essential contributions one should mention the application of the operator product expansion, as was suggested by Parisi. Recently a proof was suggested for absence of renormalon singularities in formula_10 theory and a general criterion for their existence was formulated in terms of the asymptotic behavior of the Gell-Mann - Low function formula_11. Analytical results for asymptotics of formula_11 in formula_10 theory and QED indicate the absence of renormalon singularities in these theories.