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Recall the following definitions. The smooth manifold $\mathbb{S}_R^n$ is defined as a hypersurface in $\mathbb{R}^{n+1}$, and if $\iota:\mathbb{S}_R^n\to\mathbb{R}^{n+1}$ is the inclusion map and $\bar{g}$ the Riemmanian metric on $\mathbb{R}^{n+1}$, then we let $\mathring{g}_R:=\iota^*\bar{g}$ be the Riemmanian metric on $\mathbb{S}_R^n$, so that $(\mathbb{S}_R^n,\mathring{g}_R)$ is a Riemannian manifold.

We also define the stereographic projections $$\varphi_\pm:\mathbb{S}_R^n\setminus\{\mp R e_{n+1}\}\to\mathbb{R}^n,\ (x^1,\ldots,x^{n+1})\mapsto\left(\frac{Rx^1}{R\pm x^{n+1}},\ldots,\frac{Rx^n}{R\pm x^{n+1}}\right)$$ which have inverses $$\varphi_\pm^{-1}:\mathbb{R}^n\to\mathbb{S}_R^n\setminus\{\mp R e_{n+1}\},\ (y^1,\ldots,y^n)\mapsto\left(\frac{2R^2 y^1}{R^2+{\vert y\vert}^2},\ldots,\frac{2R^2 y^n}{R^2+{\vert y\vert}^2},\pm\frac{R^3-R{\vert y\vert}^2}{R^2+{\vert y\vert}^2}\right)$$ which allows us to define coordinates $(y_\pm^1,\ldots,y_\pm^n)$ on $\mathbb{S}_R^n\setminus\{\mp R e_{n+1}\}$, though we will typically write $y^j$ in place of $y_\pm^j$ for convenience. If we let $g=\mathring{g}_R$, then a direct computation tells us that $$g_{ij} = \frac{4R^4\delta_{ij}}{(R^2+{\vert y\vert}^2)^2}$$ in coordinates. The formulae for the Chritsoffel symbols are only slightly more complicated. In coordinates, we have $$\Gamma_{ij}^k = -\frac{2y^i}{R^2+{\vert y\vert}^2}\ \mathrm{if}\ j=k,\ \Gamma_{ij}^k = -\frac{2y^j}{R^2+{\vert y\vert}^2}\ \mathrm{if}\ i=k$$ while $$\Gamma_{ij}^k = \frac{2y^k}{R^2+{\vert y\vert}^2}\ \mathrm{if} i=j\ne k$$ and $\Gamma_{ij}^k=0$ otherwise. Now, we can express the Riemannian curvature tensor $\mathrm{R}$ in coordinates as $$\mathrm{R}_{ijk}^m = \Gamma_{ik}^l\Gamma_{jl}^m - \Gamma_{jk}^l\Gamma_{il}^m + \partial_j\Gamma_{ik}^m - \partial_i\Gamma_{jk}^m.$$

Unlike with previous computations, however, I see of no obvious simplification that can be made to this expression. So this is my question: is there a simple formula for $\mathrm{R}$ that works for all $n$? Is there some simplification that can be made to reduce the possible forms of $\mathrm{R}_{ijk}^m$ to a small number of cases? Or do we simply have increasingly convoluted and messy expressions for $R$ as $n\to\infty$?

For that matter, is there a simpler way of expressing $\mathrm{Ric}_{ij}=\mathrm{R}_{kij}^k$, or $K = g^{ij}\mathrm{Ric}_{ij} = g^{ik}\mathrm{R}_{ijk}^j$?

It may possibly be useful to note that $$ \sqrt{\det(g_{ij})}=\left(\frac{2R^2}{R^2+\lvert y\rvert^2}\right)^n $$

It may also possibly be useful to note that

$$ \Delta f = -\left(\frac{R^2+\lvert y\rvert^2}{2R^2}\right)^2\sum_{j=1}^n\left(\frac{\partial^2f}{(\partial y^j)^2} - \frac{2(n-2)y^j}{R^2+\lvert y\rvert^2}\frac{\partial f}{\partial y^j}\right) $$ where $\Delta$ is the Laplace-Beltrami operator.

  • What do you mean by "in this full generality"? What level of generality? From the rest of your question I thought you were referring to a specific Riemannian manifold. – Chill2Macht Mar 09 '17 at 19:12
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    I mean, for all $n$. The question is clearly trivial if $n=1$, and is tractable for $n=2$. However, I was wondering if there's a simple formula that works in all dimensions. – Dominic Wynter Mar 09 '17 at 19:14
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    Since $g$ is conformal to the Euclidean metric, you can use the general formulas for curvature of a conformally related metric. See also this MSE post. – Jack Lee Mar 09 '17 at 20:06
  • The coefficients of the Riemann tensor satisfy several symmetry relations, which cut down the computation a bit. But in any case, the computation of $R_{i,j,k}^m$ is not intractable in any sensible sense: the computation is more or less independent of the dimension. Have you actually tried to compute those coefficients? – Mariano Suárez-Álvarez Jun 21 '17 at 06:05
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    For the sphere, you can do a calculation for every $n$ a few different ways - there's a method of foliations using distance functions and level sets of distance functions to build up over the dimensions that Petersen uses in his book to compute it. In general, it gets harder to compute the curvature tensor as $n$ gets large though for other manifolds that don't simple descriptions. – A. Thomas Yerger Jun 21 '17 at 16:11

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