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Let $({M},g)$ be a compact Riemannian manifold with Levi-Civita connection $\nabla$. It is well known that the Poisson equation $$\Delta f=S$$

where $\Delta:C^{\infty}({M})\to C^{\infty}({M})$ denotes the Laplace-Beltrami operator, has a solution on $C^{\infty}({M})$ with some source $S\in C^{\infty}({M})$ if and only if

$$\int_{{M}}S\;\mathrm{vol}_{g}=0.$$

Furthermore, this solution is unique up to adding a constant, since the only harmonic functions on a compact manifolds are the constant functions.

Now, my question is, is there any literature about the more general case of a Poisson equation acting on tensor fields? More explicitely, under which conditions does the equation

$$\Delta T=S$$

on $\Gamma^{\infty}(T^{\ast}{M}^{\otimes k})$ for some source $S\in\Gamma^{\infty}(T^{\ast}{M}^{\otimes k})$ have a solution, where $\Delta=\mathrm{tr}_{g}(\nabla^{2})$ in this case denotes the connection Laplacian.

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    $\begingroup$ You can safely use the Fredholm alternative. The existence and uniqueness of the solution (up to regularity issues) is guaranteed by the Lax-Milgram theorem. $\endgroup$ Commented Mar 6, 2023 at 11:08
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    $\begingroup$ The Laplacian over tensors is also a symmetric operator. Denote by $H_k\subset C^\infty (T^*M^{\otimes k})$ the space of harmonic tensors. Fredholm alternative states that the equation $\Delta T=S$ has a solution iff $S$ is $L^2$-orthogonal to $H_k$. For details see Sec. 10.4.1 of www3.nd.edu/~lnicolae/Lectures.pdf $\endgroup$ Commented Mar 6, 2023 at 11:11
  • $\begingroup$ Thank you very much! $\endgroup$
    – B.Hueber
    Commented Mar 6, 2023 at 11:20

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