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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.

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 …
Noah Stein's user avatar
  • 8,501
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 …
Noah Stein's user avatar
  • 8,501
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} …
Noah Stein's user avatar
  • 8,501
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 $i \in \{0,1,\ldots,\lceil\log_2 n\rceil-1\}$ …
Noah Stein's user avatar
  • 8,501
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 …
Noah Stein's user avatar
  • 8,501
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 …
Noah Stein's user avatar
  • 8,501