Cartan pair
# Cartan pair

In the mathematical fields of Lie theory and algebraic topology, the notion of Cartan pair is a technical condition on the relationship between a reductive Lie algebra ${\mathfrak {g}}$ and a subalgebra ${\mathfrak {k}}$ reductive in ${\mathfrak {g}}$.

A reductive pair $({\mathfrak {g}},{\mathfrak {k}})$ is said to be Cartan if the relative Lie algebra cohomology

    $H^{*}({\mathfrak {g}},{\mathfrak {k}})$

is isomorphic to the tensor product of the characteristic subalgebra

    $\mathrm {im} {\bigS({\mathfrak {k}}^{*})\to H^{*}({\mathfrak {g}},{\mathfrak {k}}){\big )}$

and an exterior subalgebra $\bigwedge {\hat {P}}$ of $H^{*}({\mathfrak {g}})$, where

* ${\hat {P}}$, the Samelson subspace, are those primitive elements in the kernel of the composition $P{\overset {\tau }{\to }}S({\mathfrak {g}}^{*})\to S({\mathfrak {k}}^{*})$,
* $P$ is the primitive subspace of $H^{*}({\mathfrak {g}})$,
* $\tau$ is the transgression,
* and the map $S({\mathfrak {g}}^{*})\to S({\mathfrak {k}}^{*})$ of symmetric algebras is induced by the restriction map of dual vector spaces ${\mathfrak {g}}^{*}\to {\mathfrak {k}}^{*}$.

On the level of Lie groups, if G is a compact, connected Lie group and K a closed connected subgroup, there are natural fiber bundles

    $G\to G_{K}\to BK$,

where $G_{K}:=(EK\times G)/K\simeq G/K$ is the homotopy quotient, here homotopy equivalent to the regular quotient, and

    $G/K{\overset {\chi }{\to }}BK{\overset {r}{\to }}BG$.

Then the characteristic algebra is the image of $\chi ^{*}\colon H^{*}(BK)\to H^{*}(G/K)$, the transgression $\tau \colon P\to H^{*}(BG)$ from the primitive subspace P of $H^{*}(G)$ is that arising from the edge maps in the Serre spectral sequence of the universal bundle $G\to EG\to BG$, and the subspace ${\hat {P}}$ of $H^{*}(G/K)$ is the kernel of $r^{*}\circ \tau$.

