According to his colleague Alexander Lubotzky, Weisfeiler was studying "the more difficult questions" of algebraic groups in "the case when the field is not algebraically closed and the groups do not split or — even worse — are nonisotropic".
Lubotzky, Phillips, and Sarnak (1988), Margulis (1988), and Morgenstern (1994) show how Ramanujan graphs can be constructed explicitly.
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