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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.
7
votes
Accepted
Families of subsets containing every singleton as an intersection
You can achieve $\lvert F\rvert = 2\lceil\log_2 n\rceil$ by using all subsets of the form $\{x\in X \vert i^{\text{th}}\text{ bit of }x\text{ is }j\}$ for $i \in \{0,1,\ldots,\lceil\log_2 n\rceil-1\}$ …
3
votes
Accepted
Joint probability distribution as functions
No. Here is a counterexample. Let $\mathcal{A} = \{1,2,3,4\}$, $\mathcal{B} = \{1,2\}$, and $f(x) = g(x) = \lceil\frac{x}{2}\rceil$. Let the joint probability mass function of $X$ and $Y$ be given …
7
votes
Accepted
Do singular values of a point set determine its shape?
If I understand you correctly, you have certain sets of points which you would like to count up to permutations of the coordinates. This is the same as counting lists of points up to permutations of …
6
votes
Estimate the rank of a vector
I'll give a rough answer whose utility might depend on to what extent you want to answer the question in theory vs. practice.
The given problem is at least morally reducible to the problem #KNAPSAC …
4
votes
Examples of Super-polynomial time algorithmic/induction proofs?
The standard proofs of Sperner's Lemma are the first thing that comes to my mind in this context. They don't have exactly the form you mentioned; in particular they're not really inductive. Nonethel …
8
votes
Accepted
Is the Binomial Expectation of Convex Function Convex in p?
Here is my original answer (see below for a better one):
Writing down
\[
g(p) = \sum_{k=0}^n h(k)\binom{n}{k}p^k(1-p)^{n-k}
\]
and differentiating twice gives
\[
g''(p) = \sum_{k=0}^n h(k)\binom{n} …