Hyperfine splitting in lithiumlike scandium

High-precision experiments on the ground state hyperfine splitting in highly charged ions performed at the GSI facilities and at the EBITs (see, e.g., Refs. [1, 2, 3]) have triggered intensive theoretical investigations of this effect. The error bars of the theoretical hyperfine splitting values are mainly determined by the nuclear magnetization distribution correction, the so-called Bohr-Weisskopf (BW) effect [4]. Since the total QED correction to the hyperfine splitting is of the same order of magnitude as the uncertainty of the BW correction, investigations of pure QED effects on the hyperfine structure in heavy hydrogenlike ions turn out to be unfeasible. In this context, it was proposed to consider a specific difference of the ground state hyperfine splitting in H-like and Li-like ions [5]. In this specific difference the uncertainty due to the BW effect is strongly reduced. Recently measurement of the ground state hyperfine splitting in lithiumlike scandium has been announced [3]. In a recent work [6] we have reconsidered the theory of this effect and improved the accuracy of corresponding theoretical prediction. The ground state hyperfine splitting of a lithiumlike ion is conveniently presented as