$D=d+w$ acting on forms

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Acting one basis $e^a$, The covariant derivatives \begin{align} D e^a = de^a + w^{a}{}_{b} \wedge e^b \end{align} where $w^{a}{}_{b}$ is the one-form spin connection.

I am having difficulties for expressions of $D X_{ab}$ where $X_{ab}$ $0$-form, $1$-form and $2$-form.

My idea for $X_{ab}$ on two form is \begin{align} D X^{ab} = dX^{ab} + w^{a}{}_{c} \wedge X^{cb} - X^{a}{}_{c} \wedge w^{c}{}_{b} \end{align} and acting on one-form is \begin{align} D X^{ab} = dX^{ab} + w^{a}{}_{c} \wedge X^{cb} + X^{a}{}_{c} \wedge w^{c}{}_{b} \end{align} and finally acting on zero-form is \begin{align} D X^{ab} = dX^{ab} + w^{a}{}_{c} X^{cb} - X^{a}{}_{c} w^{c}{}_{b} \end{align}

The followings are my reasoning

Recall Cartan's structure formula \begin{align} &T^a = De^a =de^a + w^a{}_{b} \wedge e^b \\ &R^{ab} = d w^{ab} + w^{a}{}_{c} \wedge w^{cb} \end{align} If we employ covariant derivatives once more, we get $D R^{ab}=0$.

\begin{align} D R^{ab} &= dR^{ab} + w^{a}{}_{c} \wedge R^{cb} - R^{a}{}_{c} \wedge w^{c}{}_{b} \\ & = (d w^{ac} \wedge w_{c}{}^{b} - w^{ac} \wedge d w_{c}{}^{b} ) + w^{a}{}_{c} \wedge ( dw^{cb} + w^{cd} \wedge w_{d}{}^{b}) - (dw^{ac} + w^{ad} \wedge w_{d}{}^{c} ) \wedge w_{cb} =0 \end{align} That's the reason why for $2$-form $X_{ab}$, I obtain

\begin{align} D X^{ab} = dX^{ab} + w^{a}{}_{c} \wedge X^{cb} - X^{a}{}_{c} \wedge w^{c}{}_{b} \end{align}

Second under the decomposition of spin-connection $w^{ab} = \mathring{w}^{ab} + \kappa^{ab}$ where $\mathring{w}$ is torsion-less part so that $de^a + \mathring{w}^{a}{}_{b} \wedge e^b=0$, and hence $T^a = \kappa^{a}{}_{b} \wedge e^b$.

In the literature, they just wrote the decomposition $R^{ab} = \mathring{R}^{ab} + \mathring{D} \kappa^{ab} + \kappa^{a}{}_{c} \wedge \kappa^{c}{}_{b}$. Here $\kappa_{ab}$ is one-form. Hence the direct computations gives \begin{align} D^2 e^a &= R^{a}{}_{b} \wedge e^b = (d w^{a}{}_{b} + w^{a}{}_{c} \wedge w^{c}{}_{b} ) \wedge e^b \\ &= \left( \mathring{R}^{a}{}_{b} + d \kappa^{a}{}_{b} + \mathring{w}^{a}{}_{c} \wedge \kappa^{c}{}_{b} + \kappa^{a}{}_{c} \wedge \mathring{w}^{c}{}_{b} + \kappa^{a}{}_{c} \wedge \kappa^{c}{}_{b} \right) \wedge e^b \\ & = \left( \mathring{R}^{a}{}_{b} + \mathring{D} \kappa^{a}{}_{b} + \kappa^{a}{}_{c} \wedge \kappa^{c}{}_{b} \right) \wedge e^b \end{align} So I guess for one-form $X_{ab}$ \begin{align} D X^{ab} = dX^{ab} + w^{a}{}_{c} \wedge X^{cb} + X^{a}{}_{c} \wedge w^{c}{}_{b} \end{align}


Is my approach correct? then How one can extend this to arbitrary forms with an arbitrary index?

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Attempt to make things compatible which one can read in various places in the internet such as this and Wikipedia.

  • The spin connection ${\boldsymbol{\omega}^a}_b={{\boldsymbol{\omega}_\mu}^a}_b\,dx^\mu$ is a $\mathfrak{so}(3,1)$-Lie-algebra valued one form. That is: for each general coordinate index $\mu$ (Greek letter), ${{\boldsymbol{\omega}_\mu}^a}_b$ is a certain $(4\times 4)$-matrix in its Lorentz indices $a,b$ (Latin letters). Alternatively, one can view ${\boldsymbol{\omega}^a}_b$ as a $(4\times 4)$-matrix of ordinary one-forms.

  • The indices of ${\boldsymbol{\omega}^a}_b$ are raised and lowered with the Lorentz metric $\eta={\rm diag}(1,1,1,-1)\,,$ $\boldsymbol{\omega}_{ab}=\eta_{ac}\,{\boldsymbol{\omega}^c}_b$, and not with the possibly curved metric $g_{\mu\nu}\,.$ Then there is anti-symmetry $\boldsymbol{\omega}_{ab}=-\boldsymbol{\omega}_{ba}\,.$

  • The coordinate basis one forms $dx^\mu$ are related to basis one forms $\boldsymbol{e}^a$ in the local Lorentz frame by $\boldsymbol{e}^a={\boldsymbol{e}^a}_\mu\,dx^\mu\,.$ A vector field $\boldsymbol{X}$ can therefore have Lorentz components $X^a=\boldsymbol{e}^a(\boldsymbol{X})$ or ordinary coordinate components $X^\mu=dx^\mu(\boldsymbol{X})\,.$ This vector field can be viewed as a vector-valued $0$-form. Its covariant derivative coming from the spin connection is therefore the Lorentz-vector valued one-form $$ DX^a=dX^a+{\boldsymbol{\omega}^a}_b\,X^b\,. $$ Applying this to the coordinate basis vector $\boldsymbol{e}_\mu$ gives $$ D_\mu X^a=\partial_\mu X^a+{{\boldsymbol{\omega}_\mu}^a}_b\,X^b\,. $$ Analogously, when $T^{ab}$ is a contravariant rank-$2$ tensor then \begin{align} D\, T^{ab}&=dT^{ab}+{\boldsymbol{\omega}^a}_c\,T^{cb}+ {\boldsymbol{\omega}^b}_c\,T^{ac}\,,\\[2mm] D_\mu\, T^{ab}&=\partial_\mu T^{ab}+{{\boldsymbol{\omega}_\mu}^a}_c\,T^{cb}+ {{\boldsymbol{\omega}_\mu}^b}_c\,T^{ac}\,. \end{align} The formulas are fully analogous to the Levi-Civita connection where the ordinary Christoffel symbols are replaced by ${{\boldsymbol{\omega}_\mu}^a}_b\,.$

  • Now we go from $0$-forms to one-forms. If $\boldsymbol{\alpha}^a={\boldsymbol{\alpha}^a}_b\,\boldsymbol{e}^b$ is a Lorentz-vector valued one-form then the covariant derivative is defined as $$ D\,\boldsymbol{\alpha}^a=d\boldsymbol{\alpha}^a+{\boldsymbol{\omega}^a}_b\color{red}{\wedge} \boldsymbol{\alpha}^b\,. $$ Same rule, just that the product in $\mathbb R$ is replaced by the $\color{red}{wedge~product}$. When $\boldsymbol{\alpha}^{ab}$ is a rank-$2$ Lorentz-tensor-valued one-form then (fully compatible with the case for $T^{ab}$) $$ D\,\boldsymbol{\alpha}^{ab}=d\boldsymbol{\alpha}^{ab}+{\boldsymbol{\omega}^a}_c\wedge\boldsymbol{\alpha}^{cb}\color{red}{+} {\boldsymbol{\omega}^b}_c\wedge\boldsymbol{\alpha}^{ac}\,. $$ When $\boldsymbol{\alpha}$ has one contravariant upper and one covariant lower index the $\color{red}{+}$ sign becomes a minus sign, or which is the same, we can swap the factors in the wedge product and keep the plus sign: $$ D\,{\boldsymbol{\alpha}^a}_b=d{\boldsymbol{\alpha}^a}_b+{\boldsymbol{\omega}^a}_c\wedge{\boldsymbol{\alpha}^c}_b+ {\boldsymbol{\alpha}^a}_c\wedge {\boldsymbol{\omega}^c}_b\,. $$

  • All of the above sign conventions are compatible with the formula in Wikipedia by which the covariant derivative of a $(1,1)$-tensor valued $p$-form is $$\boxed{\quad\phantom{\Bigg|} D{V^a}_b=d{V^a}_b+{\boldsymbol{\omega}^a}_c\wedge {V^c}_b-(-1)^p\,{V^a}_c\wedge {\boldsymbol{\omega}^c}_b\,.\quad} $$ When $V$ has two upper indices (like your $X$) we should have $$\boxed{\quad\phantom{\Bigg|} DV^{ab}=dV^{ab}+{\boldsymbol{\omega}^a}_c\wedge V^{cb}\color{red}{+}(-1)^p\,V^{ac}\wedge {\boldsymbol{\omega}^b}_c\,.\quad} $$ Swapping the wedge products allows to get rid of the $(-1)^p\,:$ $$\boxed{\quad\phantom{\Bigg|} D{V^a}_b=d{V^a}_b+{\boldsymbol{\omega}^a}_c\wedge {V^c}_b- {\boldsymbol{\omega}^c}_b\wedge {V^a}_c\,.\quad} $$ $$\boxed{\quad\phantom{\Bigg|} DV^{ab}=dV^{ab}+{\boldsymbol{\omega}^a}_c\wedge V^{cb}\color{red}{+} {\boldsymbol{\omega}^b}_c\wedge\,V^{ac}\,.\quad} $$ In that format the signs only depend on the index placements and not what rank $p$ that form $V$ has.