Lie algebras of quotient groups

We give conditions on a diffeological group $G$ and a normal subgroup $H$ under which the quotient group $G\mathrm{/}H$ differentiates to a Lie algebra for which $\mathrm{Lie}(G\mathrm{/}H)\cong \mathrm{Lie}(G)\mathrm{/}\mathrm{Lie}(H)$. Our Lie functor is instantiated by the tangent structure on elastic diffeological spaces introduced by Blohmann. The requisite conditions on $G$ and $H$ hold, for example, when $G$ is a convenient infinite-dimensional Lie group and $H$ is countable, or when $G$ is finite-dimensional and $H$ is arbitrary. To recognize that convenient infinite-dimensional manifolds are elastic diffeological spaces, we give a characterization of convenience in terms of the diffeological tangent functor: a separated and bornological locally convex topological vector space $E$ is convenient if and only if the natural map $E\times E\to TE$ is an isomorphism of diffeological spaces. As an application, we integrate some classically non-integrable Banach-Lie algebras to diffeological groups.