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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.

16 votes
0 answers
976 views

A Combinatorial Game: the Snake and the Hunter

The Snake and the Hunter is a game for two players who play in two rounds interchanging the roles of snake and hunter. The game is played in a rectangular grid of points, say $6 \times 6$. In both rou …
Bernardo Recamán Santos's user avatar
15 votes
2 answers
882 views

Lattice n-gons with ordered side lengths 1,2,3,...,n

Consider the octagon in the Cartesian plane with vertices at (0,0), (1,0), (1,2), (4,2), (4,6), (7,2), (7,8), and (0,8). Are there other (infinitely many) polygons, such as this, lying entirely in the …
Bernardo Recamán Santos's user avatar
9 votes
2 answers
1k views

A property of 47 with respect to partitions into five parts

Is 47 the largest number which has a unique partition into five parts (15, 10, 10, 6, 6), no two of which are relatively prime?
Bernardo Recamán Santos's user avatar
9 votes
1 answer
250 views

Finding the largest number which cannot be the sum of the labels of the Petersen graph

The vertices of the Petersen graph (or any other simple graph) can be labelled in infinitely many ways with positive integers so that two vertices are joined by an edge if, and only if, the correspond …
Bernardo Recamán Santos's user avatar
9 votes
1 answer
348 views

A Combinatorial Game with Integer Sequences

Two players, Alice and Bob, take turns constructing a sequence $a_1,a_2,a_3,\dots$, of distinct positive integers, none greater than a given parameter $K$. Alice plays first and makes $a_1=1$. Thereaf …
Bernardo Recamán Santos's user avatar
7 votes
3 answers
977 views

Tiling a square with rectangles whose areas or perimeters are 1, 2, 3, ..., N

For which positive integers N does there exist a square that can be completely tiled with N rectangles of integer sides whose areas or perimeters are precisely 1, 2, 3, ..., N?
Bernardo Recamán Santos's user avatar
6 votes
6 answers
1k views

Least number of vertices in a graph with which one can uniquely recover some partition of N

Given a partition of an integer $N$, its $P$-graph is the graph whose vertices are its parts, two of which are joined by an edge if and only if they have a common divisor greater than one (i.e. they a …
Bernardo Recamán Santos's user avatar
5 votes
0 answers
155 views

Tiling rectangles using all squares of sides 1, 2, 3, ..., n

Do integers n greater than 2 exist such that all the squares of sides 1, 2, 3, ..., n can be partitioned into two or more sets (none a singleton) each of whose squares can be used to tile a rectangle? …
Bernardo Recamán Santos's user avatar
5 votes
1 answer
247 views

Dealing cards numbered $1$ to $n$ into piles

Is anything known about the following? I hold in my hand a shuffled pack of cards numbered $1$ to $n$. One by one, I place them all, face up, on a table in piles. For each card I deal from my hand, sa …
Bernardo Recamán Santos's user avatar
4 votes
0 answers
145 views

Tiling squares with oblongs

An oblong is a rectangle whose width and length are consecutive integers: 1x2, 2x3, 3x4, etc. Does N exist such that it is possible to split the first N oblongs into 2 or more non-intersecting sets so …
Bernardo Recamán Santos's user avatar
4 votes
0 answers
229 views

A property of the partitions of 311 with regard to the divisors of its parts

Given a multiset of positive integers, its P-graph is the loopless graph whose vertex set consists of those integers, any two of which are joined by an edge if they have a common divisor greater than …
Bernardo Recamán Santos's user avatar
4 votes

Least number of vertices in a graph with which one can uniquely recover some partition of N

Freddy Barrera has shown that $k(1000)>5$ by verifying that every graph with fewer than 6 vertices (other than the singleton) is the P-graph of at least two partitions of 1000. On the other hand, from …
Bernardo Recamán Santos's user avatar
2 votes
0 answers
229 views

Generating all graphs of order 4 with the help of Collatz

Given a set of positive integers, its common divisor graph ( CD-graph) is the graph whose vertices are the integers, two of which are joined by an edge if (and only if) they have a common divisor grea …
Bernardo Recamán Santos's user avatar
2 votes
1 answer
212 views

Are there graphs for which infinitely many numbers cannot be the sum of the labels of its ve...

The vertices of any simple graph can be labeled in infinitely many ways with positive integers so that two vertices are joined by an edge if, and only if, they have a common divisor greater than 1. …
Bernardo Recamán Santos's user avatar
2 votes
2 answers
225 views

Generating all pentominoes by cutting and pasting

Is it possible to place the twelve pentominoes around a circle in such a way that if two of the pentominoes find themselves next to each other, it is because one of the two can be obtained from the ot …
Bernardo Recamán Santos's user avatar

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