“…Many codes have been developed in the last 30 years (Machetel et al, 1986;Glatzmaier, 1988;Bercovici et al, 1989;Zhang and Christensen, 1993;Zhang and Yuen, 1995;Ratcliff et al, 1996;Iwase, 1996;Zhong et al, 2000;Tabata and Suzuki, 2000;Richards et al, 2001;Kageyama and Sato, 2004;Yoshida and Kageyama, 2004;Choblet et al, 2007;Tackley, 2008;Davies et al, 2013;Kronbichler et al, 2012;Burstedde et al, 2013), and spherical shell numerical benchmarks have been carried out (Zhong et al, 2000;Stemmer et al, 2006;Zhong et al, 2008;Arrial et al, 2014). Semianalytical Stokes flow solutions derived via propagator matrix methods have also been proposed in the past (Busse, 1975;Busse and Riahi, 1982;Hager and O'Connell, 1981;Richards and Hager, 1984), while Tosi and Martinec (2007) derived a semi-analytical solution in the case of two eccentrically nested spheres.…”