Convergence estimates for the Magnus expansion IIA. $2\times 2$ matrices with operator norm

We review and provide simplified proofs related to the Magnus expansion, and improve convergence estimates. Observations and improvements concerning the Baker–Campbell–Hausdorff expansion are also made. In this Part IIA, we investigate the case of $2\times 2$ matrices with respect to the operator norm. We consider norm estimates and minimal presentations in terms of the Magnus and BCH expansions. Some results are obtained in the complex case, but a more complete picture is obtained in the real case.