Questions tagged [combinatorial-designs]

For questions about Combinatorial design theory, a part of combinatorial mathematics that deals with the existence, construction and properties of systems of finite sets whose arrangements satisfy generalized concepts of balance and/or symmetry. The theory has applications in the area of the design of experiments.

Combinatorial design theory is the part of combinatorial mathematics that deals with the existence, construction and properties of systems of finite sets whose arrangements satisfy generalized concepts of balance and/or symmetry. The theory has applications in the area of the design of experiments.

A combinatorial $t$-$(v,k,\lambda)$ design $D$ is a set of $k$-element subsets of $\{1,\ldots,v\}$ such that each $t$-element subset of $\{1,\ldots,v\}$ is contained in exactly $\lambda$ elements of $D$. Of particular interest are Steiner systems, which are designs with $\lambda = 1$.

This tag should not only be used for narrow sense combinatorial designs, but also for related combinatorial questions like packing designs, covering designs, the spatial arrangement of entries in an array as in Sudoku grids etc.

315 questions
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For which $n\in\Bbb N$ can we divide $\{1,2,3,...,3n\}$ into $n$ subsets each with $3$ elements such that in each subset $\{x,y,z\}$ we have $x+y=3z$?

For which $n\in \mathbb{N}$ can we divide the set $\{1,2,3,\ldots,3n\}$ into $n$ subsets each with $3$ elements such that in each subset $\{x,y,z\}$ we have $x+y=3z$? Since $x_i+y_i=3z_i$ for each subset $A_i=\{x_i,y_i,z_i\}$, we have $$4\sum…
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How do you create projective plane out of a finite field?

I have heard and read unclear mentions of links between projective planes and finite fields. Is it possible to construct a projective plane (or a Steiner system) starting out with a field? Could you, for example, construct the Fano plane with help…
Džuris
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How many rounds would it take to get each pair on the same team at least once, not using all possible teams?

I have a young group of kids ($30$) playing soccer and they need to be put into $6$ teams of $5$ players for each round of matches. All $6$ teams play at the same time on adjoining fields. If I wanted each kid to play on a team with every other kid…
Josh
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How many meetings would it take for 12 people to meet in 4 groups of 3 until they met everyone?

I have a group of 12 people that I would like to meet in four groups of three each month. How many minimum months would it take such that each person has been in at least one group with every other person? Below is the brute force method I used to…
Dave Holly
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"Math Lotto" Tickets - finding the minimum winning set

"Math lotto" is played as follows: a player marks six squares on a 6x6 square. Then six "losing squares" are drawn. A player wins if none of the losing squares are marked on his lottery ticket. 1)Prove that one can complete nine lottery tickets…
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Getting everyone to meet everyone else

There are 25 students in a class who sit in five rows of five. Each week they sit in a different order. After a number of weeks every student has sat next to every other student, next meaning side by side, one behind the other, or sitting diagonally…
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Question about members in sets

Let $A_1,A_2,...,A_n$ be sets with $k$ members in $A_i$ for every $1\le i\le n$. Suppose that the $A_i$ satisfy: 1) $|A_i\cap A_j| = 1$ for all $i\ne j$, 2) $A_1\cap A_2\cdots\cap A_n =\emptyset$. What is the largest $n$ for every $k\in…
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Minimal number of animals in a matching card game

I saw a card game designed for small children. Each card has a picture of 6 animals on it, and there are 31 cards. When any two cards are compared to each other, they share exactly one animal. The game is for the child to find the animal that is…
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Is there a memorable solution to Kirkman's School Girl Problem?

Given a solution to Kirkman's School Girl Problem, it is of course easy enough to check that it actually is a solution. But how could you reconstruct it if you lost it? Is there a method or algorithm for constructing a solution which is easier to…
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Coloring a Generalized Latin Square

Suppose we have an $n \times n$ array, and there is a decomposition $\mathcal{A}$ of its coordinates $a_{i,j}$ into sets $A_m$ as follows: If $a_{i,j} \in A_m$, then $a_{j,i} \in A_m$. So they're symmetrical pieces of the array (across the…
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Trying to Solve Math Problem for Real World Use - Combinatorics

I'm trying to solve a math problem that hasn't been solved - to anyone's knowledge - in the community it's being used in. I am sure it is not difficult, but I am not smart enough to figure it out. In England, when on a country shoot (part of…
Rand
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Symmetries of combinatorial structures.

Studying the automorphism groups of graphs/finite geometries/designs has been quite useful and important for both group theory and combinatorics. I know of the following books which cover the ideas involved in the role of group theory in…
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How many combinations of groups are there where no member of a group has been with another member before?

I found this hard to word in the title, so let me give an example. I have 16 students, and I want to split them up into 4 groups of 4. However, I want to make sure that every time I have a new combination of groups, that a student has never been…
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Partition a set into g groups, k different ways, such that no pair of elements is ever in the same group together more than M times

Over at Wolves and Sheep on puzzling.stackexchange.com, noedne's answer involves repeatedly partitioning a group of 99 sheep into a series of "test groups" such that All but one sheep are tested six times. No pair of sheep shares more than one…
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Who conjectured that there are only finitely many biplanes, and why?

This question on MathOverflow motivates me to ask what the reasoning is behind the conjecture that there are only finitely many biplanes. More generally, it has been conjectured that for fixed $\lambda>1$ there are only finitely many triples…
Will Orrick
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