Scale- and scheme-independent extension of Pade approximants; Bjorken   polarized sum rule as an example
  A renormalization-scale-invariant generalization of the diagonal Pad\'e approximants (dPA), developed previously, is extended so that it becomes renormalization-scheme-invariant as well. We do this explicitly when two terms beyond the leading order (NNLO, $\sim \alpha_s^3$) are known in the truncated perturbation series (TPS). At first, the scheme dependence shows up as a dependence on the first two scheme parameters $c_2$ and $c_3$. Invariance under the change of the leading parameter $c_2$ is achieved via a variant of the principle of minimal sensitivity. The subleading parameter $c_3$ is fixed so that a scale- and scheme-invariant Borel transform of the resummation approximant gives the correct location of the leading infrared renormalon pole. The leading higher-twist contribution, or a part of it, is thus believed to be contained implicitly in the resummation. We applied the approximant to the Bjorken polarized sum rule (BjPSR) at $Q^2_{\rm ph}=5$ and $3 GeV^2$, for the most recent data and the data available until 1997, respectively, and obtained ${\alpha}_s^{\bar MS}(M_Z^2)=0.119^{+0.003}_{-0.006}$ and $0.113^{+0.004}_{-0.019}$, respectively. Very similar results are obtained with the Grunberg's effective charge method and Stevenson's TPS principle of minimal sensitivity, if we fix $c_3$-parameter in them by the aforementioned procedure. The central values for ${\alpha}_s^{\bar MS}(M_Z^2)$ increase to 0.120 (0.114) when applying dPA's, and 0.125 (0.118) when applying NNLO TPS. 
G. Cvetic and R. Koegerler