The fact that you are not even addressing the simple differential geometry facts I mentioned in post #129 could be misunderstood as a weakness, I myself wonder why you guys avoid it.
If you prove something wrong, it doesn't matter what other facts you bring to the table. Gauss-Bonnet and the fact that the geometry of the horospheres is Euclidean means that they could not possibly be topological spheres. You are claiming that they have positive curvature and are Euclidean. That is impossible. I guess maybe you are trying to say extrinsic curvature with respect to hyperbolic space, but we tried to confront you about that earlier and you would not explain what you meant. But, it doesn't matter. Gaussian curvature is 0, you integrate it over the horosphere, and you get 0. So, it's not a topological sphere, end of discussion. It doesn't matter if it has positive curvature in some other bizarre sense. It has 0 curvature in the intrinsic sense, so Gauss-Bonnet does apply.
But let's ignore that for the moment.
It says in WP:"In Riemannian geometry, a Bryant surface is a 2-dimensional surface embedded in 3-dimensional hyperbolic space with constant mean curvature equal to 1."
I assume this means mean curvature with respect to hyperbolic space.
Horospheres are the only Bryant surfaces in which all of the surface points are umbilical points.
I'm not familiar with that fact, but it could be right.
And it is a well known fact of differential geometry that only for umbilical points the gaussian curvature equals the square of the mean curvature at that point,
Nope. That is for surfaces in R^3. An umbilic point is where the principal curvatures are equal. So the mean curvature equals the principal curvatures. So, with respect to an embedding in Euclidean space, the mean curvature equals Gaussian curvature. But the Gaussian curvature can't be computed in the same way with respect to an embedding in hyperbolic space--the Gaussian curvature is the product of principal curvatures with respect to an embedding in R^3. That is your mistake.
as a conclusion horospheres have positive gaussian curvature and therefore gauss-bonnet theorem is fine with them.
If it were so, then you would have derived a contradiction and destroyed the whole of mathematics.
Do you accept that horospheres are objects in H^3?
Yes, and therefore, they are missing a point because the whole sphere has a point in the boundary of the closed ball, which is not in H^3.
Do you agree that a horosphere has a extrinsic positive curvature in H^3?
What do you mean?
Do you recognize the formula that relates extrinsic mean curvature and gaussian curvature for submanifolds that are totally umbilical?
Yes, but for surfaces in R^3.
Do you admit horospheres are totally umbilical surfaces?
I think so.
Are you able to leave for a moment the Euclidean ambient mindframe?
Yes, but that is irrelevant. If you prove something using the Euclidean mindframe, it is still correct regardless of whether you switch to a different mindframe.
Do you understand that manifolds and submanifolds are different objects with respect to their Riemannian metric?
Unclear statement. Usually, you use the same metric, but you restrict it to the submanifold. But the geometry can be different, yes.
Do you see for example that a torus has 0 total curvature and yet when embedded in Euclidean space doesn't have euclidean metric (the external sides have round metric and the internal hyperbolic metric)
Yes, I was the one trying to tell YOU that earlier.
or that a Clifford torus embedded in S^3 has round metric and that doesn't make it a topological sphere?
A torus can't have a "round" metric, by which I think you mean positive curvature (it may have positive curvature in some places).
Why can't you understand that a topological sphere like the horosphere embedded in hyperbolic sphere can have a Euclidean metric due to its ambient space?
Because that contradicts Gauss-Bonnet. When you take the point out, yes, it can have a Euclidean metric. But you keep insisting it's not Euclidean (in contradiction to yourself) since it has positive curvature.
Do you see that a globally Euclidean metric means 0 gaussian curvature for manifolds and for submanifolds embedded in Euclidean space?
Not just Euclidean space. The Gaussian curvature is intrinsic, so it does not depend on the embedding:
"Gauss's Theorema Egregium (Latin: "remarkable theorem") states that Gaussian curvature of a surface can be determined from the measurements of length on the surface itself. In fact, it can be found given the full knowledge of the first fundamental form and expressed via the first fundamental form and its partial derivatives of first and second order. Equivalently, the determinant of the second fundamental form of a surface in R3 can be so expressed. The "remarkable", and surprising, feature of this theorem is that although the definition of the Gaussian curvature of a surface S in R3 certainly depends on the way in which the surface is located in space, the end result, the Gaussian curvature itself, is determined by the inner metric of the surface without any further reference to the ambient space: it is an intrinsic invariant. In particular, the Gaussian curvature is invariant under isometric deformations of the surface."
http://en.wikipedia.org/wiki/Gaussian_curvature
Do you admit the gauss-bonnet theorem?
What an odd thing to ask when you are the one denying it.
Can you calculate the Euler characteristic of the horosphere taking into account that it has positive gaussian curvature?
It does not have positive curvature.
Did you understand that this gaussian positive curvature is derived from the fact that a horosphere is a submanifold with positive mean curvature and totally umbilical?
I understand that you said it, and I understand that you were incorrect