Martin M. W.
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How do pure mathematicians assess whether their research ambitions can be realistically achieved?
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89 votes

Over decades, and across multiple research fields, I've noticed a way to predict I'm on track to make progress. I discover something interesting, only to learn it is already known. As a student, this ...

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Is $\mathbb{R}^3 \setminus \mathbb{Q}^3$ simply connected?
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66 votes

Yes, the complement of any countable set in $\mathbb{R}^3$ is simply connected, by the Baire category theorem. Say your set is $X = \{x_1, x_2, ... \}$, and let $y$ be any point in $\mathbb{R}^3 \...

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Fundamental Examples
60 votes

For a picture that launched a thousand papers, I'd nominate the bifurcation diagram of the logistic map. (image via Wikipedia). Answered by Martin M. W.

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Deep learning / Deep neural nets for mathematician
26 votes

Update The Coursera course I recommended long ago has now gone offline, although you can find links to the slides and videos on Hinton's home page. In any case, the field has continued to advance ...

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Examples where Kolmogorov's zero-one law gives probability 0 or 1 but hard to determine which?
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26 votes

There's a set of good examples from percolation theory: http://en.wikipedia.org/wiki/Percolation_theory If you create a "random network" with a certain probability p of edges between nodes (see ...

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Seifert surfaces of torus knots
24 votes

This picture--for a (7,2) torus knot--shows a geometric pattern you can extend to any (n,2) torus knot. The image is part of Figure 18 in the visually rich paper by Jarke van Wijk and Arjeh Cohen: "...

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How to transition from pure math PhD to nonacademic career?
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23 votes

This is the perspective of someone who went from a math PhD to the media industry and then to software/tech (and enjoyed it immensely). I can't speak for finance. Don't spend time on classes; instead,...

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What is the smallest set of real continuous functions generating all rational numbers by iteration?
21 votes

You only need one continuous function. There exists a continuous function $f: \mathbb{R} \to \mathbb{R}$ with a dense orbit, according to this MathOverflow answer. As in Saúl's construction, you can ...

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How large can you draw an island on a map?
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21 votes

Similar issues come up in studying gerrymandering (drawing political districts with partisan objectives), where it's useful to have a measure of how "irregular" a region is. You can read about ...

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Are there any interesting connections between Game Theory and Algebraic Topology?
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17 votes

One example is in the concept of a Nash equilibrium, whose existence can be proved using various (topological) fixed point theorems. (Google "nash equilibrium proof" for a wide variety of examples... ...

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Patterns in Generalized Continued Fractions
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15 votes

I think under any reasonable definition there will be only countably many explicit formulas or patterns. So in fact most reals can't be expressed this way. (See also the concept of "computable number."...

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Definition of a strange attractor.
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15 votes

This is a good question. For some reason, terminology in dynamical systems is not standardized at all--and it's interesting to disentangle various definitions. A good book to look at is Differential ...

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How to understand a solenoid?
11 votes

You can visualize a solenoid as part of a simple 3D dynamical system. The Wikipedia page on solenoids has excellent illustrations. If you want something even more concrete, and have some "Silly Putty" ...

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Is it alright for STD error bars to be below zero?
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11 votes

Your error bars may be giving you a hint to look more closely at the distribution of your data: it may not be symmetric. For example, if your data is essentially log-normal you could work with the ...

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Foliation with leaves which are and are not dense
11 votes

No, there doesn't exist such a foliation. The existence of any foliation would mean the Euler characteristic is zero, so the surface must be either a torus or a Klein bottle. Foliations for these ...

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Does the Baker-Campbell-Hausdorff formula hold for vector fields on a (compact) manifold?
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11 votes

To answer your first question, the composition of two time-1 flows won't necessarily be another time-1 flow. One way to see this is to note that when a time-1 flow $\phi_X$ has a periodic point $P$ (...

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How to construct a topological conjugacy?
10 votes

Your intuition is on target, since you mention the Banach fixed point theorem together with the Hartman-Grobman theorem. The proofs I've seen of Hartman-Grobman find the topological conjugacy as the ...

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A regularity property of transition matrices for the cat map
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9 votes

The reason that the matrices are alike is that, up to a multiplicative factor, each is approximately encoding $Area(T(R_i) \cap R_j))$. This is because the rational points with denominator $q$ are ...

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Quantitative versions of ergodic theorem
9 votes

For the specific case you mention of an irrational rotation of the circle, it depends on the rotation number $r$. You can get slow asymptotic convergence for something like $$r=\sum{10^{-n!}}$$ (and ...

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Potential connected non-Lie subgroup
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7 votes

I think the answer is, yes, the graph can be connected. By definition, if the graph G is not connected, then we can find disjoint nonempty open sets A and B, such that G is contained in A union B. In ...

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Dynamical properties of injective continuous functions on $\mathbb{R}^d$
6 votes

This is an observation, along with a sketch of a potential construction where (1) does not imply (2) for $\mathbb{R}^3$. Observation. I want to point out an example of a closed set $X \subset \mathbb{...

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Combinatorial distance ≡ Euclidean distance
5 votes

In all dimensions n>2, you get a non-regular polytope with this property if you take two simplices and glue two faces together. Pairs of vertices will be 1 unit apart, topologically and geometrically, ...

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How to smootly interpolate between möbius transformations?
5 votes

By now you may have more ideas than you need, but heck, here's another non-matrix viewpoint. In the disk model, you can identify your transformations with lists of three distinct points $(a,b,c)$ on ...

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Force-directed graph drawing in 1D?
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4 votes

I think this question might be better for one of the computer science StackExchange sites. In any case, the literal answer to your question is No. The trouble is that in a conventional force-directed ...

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Global stability for dynamical systems in $R^n$
4 votes

As the questioner notes in a comment, the answer is Yes for n<3. One way to create counterexamples for larger n is to use the work on the Seifert Conjecture. Start with a vector field pointing ...

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Searching global minima fast?
4 votes

You need more information to do anything useful. (An upper bound on the global minimum isn't very special--you can sample your function at any point to get one.) Without additional restrictions on ...

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The Arnold cat map
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4 votes

Unless I'm misinterpreting the question, the SRB measure is just Lebesgue measure... the cat map is hyperbolic, preserves area, and is topologically transitive. See Theorem 3.10 and the following ...

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Are $C^1$ vector fields generating an ergodic flow $C^0$ dense?
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3 votes

You can't always approximate by ergodic flows, because ergodic flows might not even exist. For example, on $S^2$ the Poincare-Bendixson theorem rules out ergodic flows, but there are many measure-...

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Existence of a continuous ergodic dynamical system for a given distribution?
3 votes

Let $\mu$ be Lebesgue measure on $S^1$, and $\delta_P$ be a point-mass at a point $P \in S^1$. Then there is no flow on $S^1$ whose time averages lead to $\frac{1}{2}(\mu + \delta_P)$. (Consider the ...

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Planar flow with bounded orbits and a single equilibrium point
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3 votes

(edited to include Willie Wong's idea for $C^0$ case.) This kind of flow can't exist in any dimension. Let $S$ be the unit sphere and $B$ be the open unit ball. If the origin is a global attractor for ...

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