Abstract We analyse the top-quark decay at next-to-next-to-leading order (NNLO) in QCD by using Principle of Maximum Conformality (PMC) which provides a systematic way to eliminate renormalization scheme and scale ambiguities perturbative predictions. The PMC scales coupling constant $$\alpha _s$$ α s<...

We investigate the renormalization group optimized perturbation theory (RGOPT) at next-to-next-to-leading order (NNLO) for thermal scalar field theory. From comparing three thus available successive RGOPT orders we illustrate efficient resummation and very good apparent convergence properties of method. In particular remnant scale dependence thermodynamical quantities is drastically improved as...