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reviewed  Approve How many labelled disconnected simple graphs have n vertices and floor((n choose 2)/2) edges? 
Jul 1 
answered  Given $x$ in a pathconnected open set $S$ on the plane, are there noncrossing paths from $x$ to every point in $\partial S$? 
Jul 1 
comment 
Given $x$ in a pathconnected open set $S$ on the plane, are there noncrossing paths from $x$ to every point in $\partial S$?
Your argument using the Riemann mapping theorem is flawed. The Riemann map is not guaranteed to extend continuously to the boundary. See Carathéodory's theorem. 
Jun 29 
revised 
Probability that random nonnegative integer matrix is singular
Switched matrices. Added OEIS information. 
Jun 23 
revised 
Why does the bitxor function appear in Nim?
deleted 1 character in body 
Jun 23 
answered  Why does the bitxor function appear in Nim? 
Jun 23 
comment 
Generalization of SpragueGrundy Theorem
I didn't notice the difference between Moore's results and this generalization until you pointed it out. I'd just cite both Moore and SpragueGrundy as I think it's an immediate consequence of these two theorems. 
Jun 22 
comment 
Sizes of maximum matchings in a finite, simple, undirected graph
Any union of copies of graphs with different sizes of maximal partial matchings works the same way. I believe if you form a partial matching randomly on a large grid, say a $2n \times m$ rectangle, adding edges until it is maximal, you miss close to a fixed percentage of the vertices with high probability. 
Jun 22 
comment 
Is $\sqrt {2 \sqrt {3 \sqrt {4 \ldots}}}$ a rational number?
The mathematical question is interesting, which is why so many people have voted it up. However, it should be stated that this seems to be an open question, and there should be links to the other places the question was recently asked. The way this question was presented with no context was poor, and that's why there are so many down votes. 
Jun 19 
comment 
Calculate the maximum probability of a result of rolling n dice of varying number of faces
This isn't researchlevel mathematics without some indication that straightforward approaches are not adequate. You can use dynamic programming: Let a[i,j] be the probability that the first j dice have a sum of i. The simple approach to calculating this takes $O(n^2k^2)$ because you calculate nk values n times, and each sum is over k terms. You can speed that up by using the fast Fourier transform, or by using the binomial theorem if dice are repeated. You could estimate the maximum values using a normal approximation instead. 
Jun 17 
revised 
Are all mixtures of these unimodal functions unimodal?
Added image 
Jun 17 
answered  Are all mixtures of these unimodal functions unimodal? 
Jun 15 
revised 
Mathematical journals (maybe in the past) with regular competitions?
Restored an omitted part of the question. 
Jun 15 
comment 
Expected length of minimum spanning trees
Don't you just get a weighted average of the values on smaller sets of vertices, weighted by the probability that many vertices appear? 
Jun 10 
comment 
Number of Nice Matrices
@Vincent: There actually are ways you can prove something has no computable formula, say if it encodes the halting problem, although I doubt something like that would work here. 
Jun 7 
comment 
Lower bound for the probability that a certain component of a Gaussian vector dominates all others
You can translate this into a probability that another Gaussian vector is in the positive orthant, $(X_1X_2, X_1X_3,...,X_1X_n)$. These probabilities have been studied. 
Jun 5 
comment 
What's the extreme value distribution of log normals?
What do you get if you take exp of a variable distributed as the extreme distribution for a normal distribution? 
May 31 
comment 
What's that shape? Inferring a 3D shape from random shadows
As Joseph O'Rourke noted, the standard term is visual hull, not shadow hull. 
May 31 
answered  What's that shape? Inferring a 3D shape from random shadows 
May 31 
comment 
A circulant coin weighing problem
When you say "partial circulant," do you require the rows to be the first $m$ rows of a circulant matrix, as assumed in cstheory.stackexchange.com/questions/27330/… or just any $m$ rows? 