Universal extension classes for $GL_2$

Abstract

In this note we give a new existence proof for the universal extension classes for $GL_2$ previously constructed by Friedlander and Suslin via the theory of strict polynomial functors. The key tool in our approach is a calculation of Parker showing that, for suitable choices of coefficient modules, the Lyndon--Hochschild--Serre spectral sequence for $SL_2$ relative to its first Frobenius kernel stabilizes at the $E_2$-page. Consequently, we obtain a new proof that if $G$ is an infinitesimal subgroup scheme of $GL_2$, then the cohomology ring $\Hbul(G,k)$ of $G$ is a finitely-generated noetherian $k$-algebra.