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Let $P\rightarrow M$ be a principal $G$-bundle with a connection 1-form $\omega$. In a local trivialisation $\tau_U \colon U\rightarrow P_U$ ($U \subset M$) we can pull the connection back to the base manifold. $$\omega_U=\tau_U^*\omega=\omega^i \otimes e_i \, .$$ Now consider the adjoint bundle $\text{ad} P$ of $P$. The principal connection induces a covariant derivative on $ad P$. In the same trivialising neighbourhood, it is given by the formula $$\nabla_X s =[\omega_U(X),s]+ds(X)$$

I am interested in the other direction. Suppose we start with a covariant derivative $\nabla=d+A$ on $\text{ad} P$. What conditions should $A$ satisfy so that there exists $\omega$ on $P$ that induces $\nabla$?

In particular, I am interested in the case when $G$ is compact, connected, semisimple. I have very little experience with Lie algebras, but I believe this should help somehow.

user313212
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balintm
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2 Answers2

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The space of derivations $\mathrm{Der}(\mathfrak g)$ consists of all linear maps $a:\mathfrak g\to\mathfrak g$ which satisfy $ a([x,y])=[a(x),y]+[x,a(y)] $ and each local section $\tau:U\to P$ induces a local trivialization $$ \varphi_\tau:U\times\mathfrak g\to\mathrm{Ad}(P)_{|U},\;\;\;\;(x,\xi)\mapsto [(\tau(x),\xi)] $$ Then for semisimple $G$ the following is true:

A covariant derivative $\nabla$ on $\mathrm{Ad}(P)$ is induced by a principal connection if and only if for each local section $\tau:U\to P$ the connection one form $A$ defined by ${\varphi^*_\tau}\nabla=d+A$ satisfies $A\in\Omega^1(U,\mathrm{Der}(\mathfrak g))$.

The reason is, that for semisimple $G$ the map $\mathrm{ad}:\mathfrak g\to\mathrm{Der}(\mathfrak g)$, $a\mapsto[a,\cdot]$ is an isomorphism, so in this case one can write $A=[\omega_U,\cdot]$ for a unique $\omega_U\in\Omega^1(U,\mathfrak g)$. One then needs to verify, that there exist a principal connection $\omega\in\Omega^1(P,\mathfrak g)$ such that $\tau^*\omega=\omega_U$ for each local section $\tau:U\to P$. For this to be true, one has to verify the transformation formula $$ \omega_U=g^*\omega_G+\mathrm{Ad}_{g^{-1}}\omega_{\tilde U} $$ where $\tilde \tau:\tilde U\to P$ is another local trivialization with $\tau=\tilde\tau g$ and $g:U\cap\tilde U\to G$ on the intersection. Since $\mathrm{ad}:\mathfrak g\to\mathrm{Der}(\mathfrak g)$ is an isomorphism, one gets the equivalent condition $$ \mathrm{ad}(\omega_U)=\mathrm{ad}(g^*\omega_G)+\mathrm{ad}\circ\mathrm{Ad}_{g^{-1}}(\omega_{\tilde U})\\ $$ Now for an arbitrary Lie group homomorphism $\lambda:G\to\tilde G$ with induced Lie algebra homomorphism $\lambda_*:\mathfrak g\to\tilde{\mathfrak g}$ one can show by unravelling the definitions and applying the chain rule that $$ \lambda_*(g^*\omega_G)=(\lambda\circ g)^*\omega_{\tilde G}\;\;\;\;\;\;\;\;\lambda_*\circ\mathrm{Ad}_{g^{-1}}=\mathrm{Ad}_{\lambda(g^{-1})}\circ\lambda_* $$ Applying these formulas for the adjoint representation $\mathrm{Ad}:G\to\mathrm{GL}(\mathfrak g)$, the transformation formula from above becomes $$ A=(\mathrm{Ad}_g)^*\omega_{\mathrm{GL}(\mathfrak g)}+\mathrm{Ad}_{\mathrm{Ad}_{g^{-1}}}\tilde A\\ =\mathrm{Ad}_{g}^{-1}d\,\mathrm{Ad}_{g}+\mathrm{Ad}_{g}^{-1}\tilde A\,\mathrm{Ad}_{g} $$ But this is precisely the transformation formula one gets for the connection $\nabla$, since the corresponding transition function for $\mathrm{Ad}(P)$ is given by $\mathrm{Ad}_{g}$.


There is also a global characterization: The bundle $\mathrm{Ad}(P)$ is fiberwise equipped with a Lie bracket $[\cdot,\cdot]\in\Gamma\Lambda^2\mathrm{Ad}(P)^*\otimes \mathrm{Ad}(P)$ by the formula $[[(s,x)],[(s,y)]]=[(s,[x,y])]$.

Then for semisimple $G$ the following is true:

A covariant derivative $\nabla$ on $\mathrm{Ad}(P)$ is induced by a principal connection if and only $\nabla [\cdot,\cdot]=0$, which means that for all $x,y\in\Gamma\mathrm{Ad}(P)$ $$\nabla[x,y]=[\nabla x,y]+[x,\nabla y]$$

To show this one can use $\varphi_\tau$ to locally identify $\mathrm{Ad}(P)$ with $U\times \mathfrak g$. Then the Lie bracket is just the Lie bracket on $\mathfrak g$ and $\nabla =d+A$. Therefore for all local sections $x,y:U\to\mathfrak g$ $$ (\nabla [\cdot,\cdot])(x,y)=\nabla[x,y]-[\nabla x,y]-[x,\nabla y]\\ =d[x,y]+A[x,y]-[dx,y]-[Ax,y]-[x,dy]-[x,Ay]\\ =A[x,y]-[Ax,y]-[x,Ay] $$ This means that $[\cdot,\cdot]$ is parallel on $U$ if and only if $A\in\Omega^1(U,\mathrm{Der}(\mathfrak g))$ and the global characterization follows from the local characterization given above.

Claire
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Let me first answer this question in more generality.

Let $E$ be a vector bundle over a base manifold $M$, then $E$ is associated to the principal bundle of linear frames $GL(E)$, and every covariant derivative on $E$ induces a connection on $GL(E)$. Indeed, taking a cover of $GL(E)$, $\{U_i,e^i\}$, where $e^i=(e^i_1,\dots, e^i_n)$ is frame for $E$ over $U_i$, and fixing a covariant derivative $\nabla$ on $E$, we can write $\nabla$ in each local frame via: $$\nabla e^i_k=\xi^a_k\otimes e^i_a$$ We can then take the matrix valued one form $\xi=(\xi^a_k)$. If $\mu_{GL}$ is the Maurer-Cartan form on $GL_n$, $\phi_i$ is the trivialization of $GL_n(E)$ induced by $e^i$, and $\pi_{GL}:U_i\times GL_n\rightarrow GL_n$, $\pi_{U_i}:U_i\times GL_n\rightarrow U_i$ are the projections, we define a connection one form on $GL(E)$ locally by: $$A|_{\pi^{-1}(U)}=\phi_i^*(\pi_{U_i}^*\xi+\pi_{GL}^*\mu_{GL})$$ These then glue together to form a global connection one form $A$ on $GL(E)$ as one can check.m

Now, if $E$ admits a $G$ structure, then there is some representation $\rho:G\rightarrow GL(\mathbb R^n)$, and a cover by trivialization such that the transition functions take values in $\rho(G)$. We can thus get a new principal bundle $G$-frame bundle $G(E)$ where the action on the frames is now given by the Lie group $G$. As an example, we can always equip $E$ with an orthogonal structure, and then obtain the bundle of orthonormal frames, $O(E)$, where the transition functions take orthonormal frames to orthonormal frames, and thus lie in the group $O(n)$. In this case, we can only lift a covariant derivative to the principal bundle $O(E)$ if, when writing the covariant derivative in an orthonormal frame, we have that $\xi^a_k+\xi^k_a=0$; in other words $\xi$ is a matrix valued one form with values in the Lie algebra $\mathfrak{o}(n)$. Note that this is an if and only if condition, if $\nabla$ induces a connection in $O(E)$, then if every orthonormal frame we have that the matrix one forms take values in $\mathfrak o(n)$.

Going back to the abstract case, suppose we have a $G$ structure on $E$, and consider the princpal $G$-frame bundle $G(E)$. We have a representation $\rho:G\rightarrow GL_n$, and this gives an induced representation $\rho_*:\mathfrak g\rightarrow \text{Mat}_{n\times n}(\mathbb R^n)$. We can now try and play the same game and claim that we can lift the covariant derivative to a connection one form if and only if the matrix valued one forms have components which lie in $\rho_*(\mathfrak g)$ for every local $G$-frame (by which I mean a frame of the form $e_i=[s,u_i]$ where $u_i$ is the standard basis vector on $\mathbb R^n$), however this does not always work. Suppose that we have a connection on $E$, and $U$ and $V$, such that $U\cap V\neq \emptyset$, with sections $s_U,s_V$ from $U,V\rightarrow P$, and local connection forms $\xi_U$ and $\xi_V$ which can be written as $\rho_*(A_U)$ and $\rho_*(A_V)$ for some $\mathfrak{g}$ valued one forms $A_U$ and $A_V$. Then it need not be the case that $A_U$ and $A_V$ glue together as $\rho_*$ may not be injective. We then need the extra condition that we can find $A_U$ and $A_V$, such that if $g:U\cap V\rightarrow G$ is the map satisfying $s_U=s_V\cdot g$, then: $$A_U=\text{Ad}_{g^{-1}}\circ A_V+g^{-1}\mu_G$$ where $\mu_G$ is the Maurer-Cartan form on $G$.

Let us now start with a principal $G$ bundle $P\rightarrow M$, then we have that the adjoint bundle is associated $\text{Ad}(P)$ is the vector bundle associated to $\mathfrak{g}$ the adjoint representation of $G$ on $\mathfrak g$. The adjoint presentation is a Lie group homomorphism $G\rightarrow GL(\mathfrak g)$, and an induced representation $\mathfrak{g}\rightarrow \text{End}(\mathfrak g)$. Fix a covariant derivative $\nabla$ on $\text{Ad}(P)$, and let $\xi$ be the local connection one form for a local $G$-frame (that is one of the form $e=[s,u]$, where $s$ is a section of $P$, and $u$ is a frame of $\mathfrak{g}$) on $\text{Ad}(P)$. Then, we can obtain a connection one form on $P$ if and only if in every such frame, for all $X\in \mathfrak{X}(U)$, and all $\Phi\in \Gamma(\text{Ad}(P),U)$ we have that: $$\xi\cdot \Phi=[\beta,\Phi]$$ for some one form $\beta$ with values in $\mathfrak{g}$, where each $\beta$ satisfies the appropriate transformation law. If the adjoint representation $\mathfrak{g}\rightarrow \text{End}(\mathfrak{g})$ is injective, then we get the compatibility condition for free so it suffices to just check that we can write $\xi\cdot \Phi$ as this in every local $G$-frame.

Edit: Note that $\text{Der}(\mathfrak{g})\subset \text{End}(\mathfrak{g})$ so if $G$ is semisimple, then $\text{ad}:\mathfrak{g}\rightarrow \text{End}(g)$ is injective as it is an isomorphism onto it's image, so Claire's answer is a special case of the above discussion.

Chris
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