In this section we discuss the finite-dimensional representation theory of the Lie algebra 𝔰𝔩2,ℂ\mathfrak{sl}_{2,\mathbb{C}}. We then use that to study the representation theory of SL2(ℂ)\operatorname{SL}_{2}(\mathbb{C}), 𝔰𝔩2,ℝ\mathfrak{sl}_{2,\mathbb{R}}, SL2(ℝ)\operatorname{SL}_{2}(\mathbb{R}), and SU(2)\mathrm{SU}(2), which are all closely related.