New posts in combinatorics

Combinatorial Interpretation of Fractional Binomial Coefficients

combinatorics binomial-coefficients generating-functions catalan-numbers

How many matrices exist with this increasing row and increasing column condition?

combinatorics matrices

Under what conditions does the n-dimensional, infinite, unit-square-grid graph contain a Hamiltonian ray?

combinatorics graph-theory

A canonical construction proving $\left|HK\right| = \left|H\right|\left|K\right| / \left|H \cap K\right|$?

combinatorics group-theory universal-property

Proving the same sum of two subsequences by Pigeonhole Principle?

sequences-and-series combinatorics pigeonhole-principle

Finding the smallest set on which a group acts faithfully

combinatorics group-theory computational-complexity permutations

Number of isoceles triangles formed by the vertices of a polygon that are not equilateral

combinatorics combinations triangles polygons

No sum of three numbers equals another number in set

combinatorics extremal-combinatorics

All clubs have a member among $n$ people

combinatorics graph-theory algorithms

Different shapes made from particular number of squares

combinatorics

Number of ways to partition $40$ balls with $4$ colors into $4$ baskets

combinatorics integer-partitions balls-in-bins

A combinatorial proof of the identity $\sum\limits_{k=0}^n\binom{2k}{k}\binom{2(n-k)}{n-k}=4^n$ [duplicate]

combinatorics

The number of esquares of idempotents in the rank 2 $\mathcal{D}$-class of $M_n(\mathbb{Z}_2)$.

linear-algebra abstract-algebra combinatorics semigroups idempotents

Number of Ways to Fill a Matrix with symbols subject to Weird contsraint.

combinatorics

More elegant proofs of $\binom a2+\binom b2\leq \binom{a+b-1}2$

combinatorics inequality binomial-coefficients combinatorial-proofs extremal-combinatorics

Asymptotics of number of ways of putting balls into bins with constraints

combinatorics asymptotics

How to group people so everyone meets?

probability combinatorics combinations

Combinatorial proof $\sum_i^{\lfloor{n/2}\rfloor} (-1)^i {n-i\choose i} 2^{n-2i} = n+1$

combinatorics combinations combinatorial-proofs

Car parking related probability

probability combinatorics

If $X$ and $Y$ are two ordered sets, how many orderings of $X \times Y$ exist that preserve the orderings of $X$ and $Y$?

combinatorics order-theory