Aff(2,R) - Knowlpedia

$\mathrm{Aff}(2,\mathbb R)$ is the Lie group of invertible affine maps $x\mapsto Ax+b$ of $\mathbb R^{2}$ , with $A\in\mathrm{GL}(2,\mathbb R)$ and $b\in\mathbb R^{2}$ . In coordinates,

(A,b)(A^{\prime},b^{\prime})=(AA^{\prime},Ab^{\prime}+b).

Dimensions and structure

This group has real dimension $6$ . The translations form a normal additive subgroup. Projection to the linear part splits by $A\mapsto(A,0)$ , giving $\mathbb R^{2}\rtimes\mathrm{GL}(2,\mathbb R)$ . In dimension one this is the ax+b group.

Direct verification

Composing the maps x ↦ Ax+b and x ↦ A′x+b′ gives x ↦ AA′x+Ab′+b. The inverse is x ↦ A⁻¹x−A⁻¹b. The underlying manifold is the product of the general linear group and the translation vector space.