# Contracting maps of hyperbolic manifolds

Is it possible for SOME positive $c$, $c<1$ to find a pair of COMPACT hyperbolic manfiolds $M^3$ and $N^3$ with a positive degree map $$f: M^3 \to N^3,$$ such that $f$ is contacting with constant $c$? Are there may examples like this?

One can ask the same question of Riemann surfaces, and it seems to me that this should be possible. For example we can take a double cover of Riemann surface with many points or ramification. Though I don't know a proof even in this case. Of course for non-ramified cover the best possible constant $c$ is $1$.

ADDED. Following the answer of Sam Need, let me give an approximative "proof" of the fact that this works in dimesnion 2. Let us triangulate a hyperbolic surface $N^2$ in triangles of very small size, that have acute angles (this is always possible). We want to show that a double cover of $N^2$ with ramifications at vertices of the triangulation will do the job. For this we need a lemma (without a proof).

Lemma. Suppose we have two hyperbolic trianlges, one very small and acute with angles $a$, $b$, $c$, and the over with angles a/2, b/2, c/2. Then there is a contacting map from the second triangle to the first one. The lemma is true, since the second trianlge will be large.

Now on the double cover we can take a trangulation that comes from $N^2$ and glue it from these triangles with half angles. Half angles come from doble cover. Then we just need to "adjust" the map.

Of course this is not a real proof, but I am 100% it can be made real.

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You should probably add hypotheses to your question, otherwise there is the trivial example mapping $\mathbb{H}^3 \to \mathbb{H}^3$ where in exponential coordinates, one takes every point closer to the origin by some factor. –  Ian Agol Dec 14 '09 at 19:00
Thanks! I corrected the question, speaking about hyperbolic manifolds I had in mind "compact" manifold –  Dmitri Dec 14 '09 at 19:20

In general, for any non-zero degree map from one closed negatively curved manifold to another, there is a canonical map (due to Besson-Courtois-Gallot) called the "natural map". However, it's only known to be pointwise volume decreasing, not necessarily contracting. They call this the "real Schwarz-Lemma". Applying the Schwarz lemma for Riemann surfaces I think gives the contracting map in this case for branched covers. Think of the induced map on the universal cover, which is the unit disk, or $\mathbb{H}^2$. The Schwarz lemma says that any conformal map from the disk to the disk is contracting, unless it's an isometry.

I thought of one (not very explicit) example in 3-D. Take two simplices in hyperbolic space. There is a canonical affine map (say in the Lorentzian model) taking one simplex to the other. This will be a contracting map for the hyperbolic metric if one simplex sits inside the other [Edit: actually I'm not sure about this now, but in the example below there exists a contracting map]. There are finitely many tetrahedra in $\mathbb{H}^3$ which give rise to fundamental domains for discrete reflection groups (see Ratcliffe). Two of these have one dihedral angle $\pi/5$, with opposite edge angle $\pi/2$ and $\pi/4$, respectively, and all other angles the same. There is a 1-parameter family of polyhedra interpolating between these (basically, just "push" the two faces closer together along the dihedral angle $\pi/5$ edge) which decreases distances. Also, the orbifold fundamental group (i.e. reflection group) from the $\pi/4$ one maps to that of the $\pi/2$ one. So there's a distance decreasing map from one orbifold to the other. Using Selberg's lemma, one may find finite-sheeted manifold covers with the same property.

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Thanks a lot for the answer! I added a proof of 2-dimensional case, but of course Schwarz lemma do the job!! I will think about your 3-dimensional example! –  Dmitri Dec 14 '09 at 20:33
After reading this I looked at the Dirichlet domains of the (n,0) fillings of the figure eight knot, using SnapPea. It looks like (a) the domains are all combinatorially identical, as n grows, and (b) they are nested. So, if my eyes are correct, your argument will also apply to branched covers over the figure eight. –  Sam Nead Dec 14 '09 at 20:47
@ Sam: As for the polyhedra, you might be interested in ams.org/mathscinet-getitem?mr=1382519. I'm not sure that there's a length-decreasing map for general polyhedra (the argument I had in mind works for a simplex). For example, there may be a very short edge near the core geodesic which increases in length in your examples as n decreases. So the natural combinatorial map of Dirichlet domains can't be length decreasing from the (nm,0) filling to the (m,0) filling. –  Ian Agol Dec 15 '09 at 0:17
"there may be a very short edge near the core geodesic which increases in length in your examples as n decreases" Ok, I see this now. My mistake. Thanks for the link to the paper. I'll take a look. –  Sam Nead Dec 15 '09 at 17:35

This question confuses me, even in dimension two. The non-trivial branched coverings $f$ I can think of are extremely contracting near the branch points. So much so that $f$ is actually expanding elsewhere to produce enough area.

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Sam, I included a "proof" of 2-dimensional case –  Dmitri Dec 14 '09 at 20:19
Ok, that seems more reasonable... –  Sam Nead Dec 14 '09 at 21:59