New posts in combinatorics

Bit hack to generate all integers with a given number of 1s

bit combinatorics

Let $S={1, 2, 3, 4, ....., 2070}$ find the number of subsets of $S$ whose sum of elements in $S$ is divisible by 9

All pairs of letters (de Bruijn sequence with $n=2,k=52$)

Gardner riddle on mathemagicians

combinatorics number-theory puzzle

In how many ways can the integers from $1$ to $n$ be divided into two groups with the same sum?

combinatorics summation integers

Count valid colourings on an hexagonal grid

sequences-and-series combinatorics discrete-mathematics graph-theory

What's the sum of the reciprocals of the numbers that can be written as the sum of two positive cubes?

real-analysis sequences-and-series combinatorics number-theory zeta-functions

Counting the number of decimals that satisfy a condition

combinatorics number-theory elementary-number-theory contest-math

Given $2^{n-1}$ subsets of a set with $n$ elements with the property that any three have nonempty intersection, prove that ....

linear-algebra combinatorics solution-verification contest-math extremal-combinatorics

How many strictly increasing functions $[4] \to [12]$ precede $(2,3,4,5)$?

combinatorics discrete-mathematics

Number of superincreasing sequences of natural numbers

Given a desired coloring scheme for a stick, how can I brush it with the fewest steps?

combinatorics algorithms puzzle

Stirling number

combinatorics stirling-numbers

Questions on proof of Konigs theorem.

combinatorics graph-theory proof-explanation

In a nearest neighbors graph (with odd number of vertices) is it possible that no vertex has degree one? [closed]

combinatorics geometry graph-theory planar-graphs

Coloring $\mathbb R^n$ with $n$ colors always gives us a color with all distances.

combinatorics contest-math combinatorial-geometry

Counting sets by their connectedness

combinatorics discrete-mathematics graph-theory computer-science generating-functions

$6$ points in the interior of a square of side length $2$.

combinatorics geometry

Determining if two Sudoku boards are in the same equivalence class

combinatorics self-learning recreational-mathematics sudoku

Does there exist integers $a, n > 1$ such that $1 + \frac{1}{1 + a} + \frac{1}{1 + 2a} + ... + \frac{1}{1 + na}$ is an integer?

combinatorics number-theory integers