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21h

comment 
Ordinals separate from set theory
Conway's surreal numbers include the ordinals. They can be developed separately from set theory, and that's what Conway did in "On Numbers and Games." Some discussion here: personal.psu.edu/t20/fom/postings/9905/msg00074.html 
Jun 22 
comment 
Flat Riemannian manifold
This doesn't seem like a researchlevel question. It might have been more appropriate for math.SE. 
Jun 22 
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Finding Riemannian metric for this geodesic
I don't understand the motivation for the question. Since no coordinate system has been specified, isn't it vacuous? For example, I could just associate the $tb^i$ with the Cartesian coordinates in a flat space, and take the given equation to be a change of coordinates. And what is the motivation for the constraint on the sum of the $\gamma$'s? This would simply seem to restrict our attention to a $(d1)$dimensional submanifold. 
Jun 21 
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What notions are used but not clearly defined in modern mathematics?
This seems to me to be a poor answer, for the same reasons I gave in a comment on the answer by Buschi Sergio. 
Jun 21 
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What notions are used but not clearly defined in modern mathematics?
This is very unclearly written and doesn't make much sense to me. For example, "Concept and existence of points, and spaces as sets of points." This sounds like a description of an axiomatic style of mathematics, in which we have certain primitive notions that we make no attempt to define explicitly. This is part of the normal process of mathematics and does not indicate that something is "not clearly defined" in the nontrivial sense of the OP's question, as I understand it. 
May 19 
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Kinematics of rolling knots
The relevant material seems to be at the end of the talk, around 9:00? One thing to think about would be whether you want to ask these questions about knots with ideal shapes, or knots that are arbitrary fattened embeddings of the topological knot in 3space. Ideal shapes are poorly characterized: arxiv.org/abs/1402.5760 . It's not obvious to me whether the dynamics are Hamiltonian. See, e.g., en.wikipedia.org/wiki/Chaplygin_sleigh . 
May 19 
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Kinematics of rolling knots
possibly related: mathoverflow.net/questions/180194/… 
May 19 
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Flat Riemannian manifold
An example that the OP might wish to think about would be a cylinder with a flat metric. That might help to clarify why the answer is framed in local terms. 
May 18 
awarded  Necromancer 
May 18 
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Diffusion on a semiRiemannian manifold?
if we lived in a space which was only semiRiemannian Huh? We do live in such a space. There is no a priori guarantee that such a space is timeorientable, has Cauchy surfaces, or lacks closed, timelike curves. You may be interested in notions from general relativity such as global hyperbolicity. See, e.g., Hawking and Ellis, The Large Scale Structure of SpaceTime, p. 206. 
May 14 
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How to learn QFT from mathematical perspective?
We have a body of math that was created for the sole purpose of understanding topic X, and which is still practiced for almost the sole reason of applying it to X. It seems odd to me to be so insistent on refusing to learn about the application to X. This is not an example like group theory, where the original motivation was much more specific (permutation groups) but now constitutes only an insignificant portion of the whole landscape. If you learn an entire field of math without understanding a single application, I would question whether you understand the field. 
May 12 
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Physics that needs “new” math
Physicist to string theorist: You've had this research program going on for 35 years, and it's not yet looking like a healthy physical theory. Why should anyone keep giving you funding? Standard answer from a string theorist: String theory is a piece of 21stcentury mathematics that's fallen out of the sky into the 20th century, and it's going to require 22ndcentury mathematics to solve it. Even if it's never going to be a correct Theory of Everything, keep funding us, because it's wonderful mathematics. 
Feb 12 
awarded  Yearling 
Feb 12 
revised 
space at the Planck scale
added 4 characters in body 
Feb 12 
answered  space at the Planck scale 
Jan 24 
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Are hyperreal numbers isomorphic to formal power series?
The product of two Laurent series need not be a Laurent series. If you want a nonarchimedean field that has welldefined multiplication and division, a good candidate is the LeviCivita field, but the LC field doesn't have $e^\omega$ where $\omega$ is infinite. It would also be odd if a structure as "natural" as the Laurent series were isomorphic to the hyperreals, since the hyperreals are nonunique in ZFC. 
Dec 29 
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Which universities teach true infinitesimal calculus?
news.slashdot.org/comments.pl?sid=99232&cid=8500301 
Dec 22 
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Why do we teach calculus students the derivative as a limit?
I've read through the whole dialog in comments about the sine function, and I'm still mystified. Among the mathematicians who got it wrong, what was the wrong answer that they gave? What was the reasoning that they offered? 
Dec 10 
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Probability that a selfavoiding walk on $\mathbb{Z}^3$ closes to a polygon
@JosephO'Rourke: I would bet you a nickel that the length before reaching a cul de sac has a distribution that drops off at least as fast as an exponential. As time goes on, the probability per unit time of being trapped should increase, because you've built up a trail that could help to trap you. Based on your simulation, I'd guess that the expected length is on the order of $10^4$. 
Dec 8 
revised 
Probability that a selfavoiding walk on $\mathbb{Z}^3$ closes to a polygon
added 18 characters in body 