Ebook: Variations on a Theorem of Tate

Let $F$ be a number field. These notes explore Galois-theoretic, automorphic, and motivic analogues and refinements of Tate's basic result that continuous projective representations $mathrm{Gal}(overline{F}/F) to mathrm{PGL}_n(mathbb{C})$ lift to $mathrm{GL}_n(mathbb{C})$. The author takes special interest in the interaction of this result with algebraicity (for automorphic representations) and geometricity (in the sense of Fontaine-Mazur). On the motivic side, the author studies refinements and generalizations of the classical Kuga-Satake construction. Some auxiliary results touch on: possible infinity-types of algebraic automorphic representations; comparison of the automorphic and Galois "Tannakian formalisms" monodromy (independence-of-$ell$) questions for abstract Galois representations.