Questions related to permutations, bijections from a finite (or sometimes infinite) set to itself.

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**1**answer

56 views

### Are certain normal matrices circulant? (Part 2)

Let $\mathcal{F}$ denote the family of real normal matrices $A$ such that $
A^TA=\begin{pmatrix}
a & b \\
b & \ddots
\end{pmatrix}$, for $b>0$.
As a user observed in the solution of Part 1 ...

**2**

votes

**1**answer

107 views

### Hopf structures on “pictorial” descriptions of permutations

There is a well-known Hopf structure on permutations, due to Malvenuto and Reutenauer. Here, the F basis elements are permutations in one-line notation, the product of two permutations is their ...

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**2**answers

412 views

### Numbers with all N-digit prefixes divisible by N

In base 10, the number 3816547290 contains every digit exactly once. When I take the first N digits, that substring is divisible by N. For example, 381 is divisible by 3, 38165 is divisible by 5, etc. ...

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**0**answers

63 views

### Name for class of flattening permutations

Let $S_n$ be the symmetric group. For any sequence of numbers $y=[y_1,y_2,\cdots,y_k]$, define the flattening operation as $\mbox{flatt}_{k}(y)$ as a relabeling of $y_1,y_2,\cdots,y_k$ in terms of ...

**12**

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**1**answer

202 views

### Number of orders of $k$-sums of $n$-numbers

Suppose we have a $n$-element set $S$. Denote the set of its $k$-element subsets by $K$ ($|K|=\binom{n}{k}$).
If the elements of $S$ are real numbers then to each $k$-element subset we can associate ...

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**2**answers

301 views

### Does a classification of simultaneous conjugacy classes in a product of symmetric groups exist?

Let the symmetric group $S_n$ on $n$ letters act on $S_n^d=S_n\times\cdots\times S_n$ by simultaneous conjugation, i.e. $\pi\in S_n$ acts on $(\sigma_1,\ldots,\sigma_d)\in S_n^d$ by ...

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**0**answers

224 views

### Polynomial dynamical systems

The question is somewhat related to the theory of permutation polynomials. Let $\mathbb{F}_p$ be a finite field of $p$ elements ($p$ is prime) and $\mathcal{V} = \mathbb{F}_p^2 = \{ (t_1,t_2)\::\: ...

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**1**answer

203 views

### Is there a geometric meaning of the Major index?

The actual question I want to ask is whether there is a geometric proof of this famous identity
$$\sum_{\sigma \in S_n} q^{\operatorname{inv} \sigma}=\sum_{\sigma\in ...

**25**

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**2**answers

818 views

### Is this graph polynomial known? Can it be efficiently computed?

I am a physicist, so apologies in advance for any confusing notation or terminology; I'll happily clarify. To provide a minimal amount of context, the following graph polynomial came up in my research ...

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**1**answer

150 views

### counting the number of ordered pairs in a permutohedron

Recall that a permutohedron is a graph on the set of permutations $S_n$ with an edge between $\sigma$ and $\tau$ if they differ by one adjacent transposition: $\tau = (i,i+1) \circ \sigma$ for some $i ...

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**0**answers

313 views

### Conjectures on fractions where each digit appears once in numerator and denominator

This is a highly redacted version of a question that was asked before. Please see Criteria of considering relevance of the question to the domain of research topics for details.
Some numerical ...

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**1**answer

164 views

### Principal Order Ideals in the Weak Bruhat Order

Let $\sigma\in S_n$ be a permutation on $n$ elements, and $\mathrm{Inv}(\sigma):=\{(i,j) : 1\leq i<j\leq n\text{ and }\sigma(i)>\sigma(j)\}$ be its set of inversions. In the weak order on ...

**10**

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**1**answer

362 views

### Categorical interpretation of disjoint cycle notation for tracing permutations

For a natural number $n\in\mathbb{N}$, let $\underline{n}$ denote the finite set $$\underline{n}:=\{1,2,\ldots,n\}.$$
A permutation $\sigma\in Aut(\underline{n})$ can be uniquely written (up to order) ...

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**5**answers

1k views

### Probability of a pair of memory cards ending up as neighbors

I am trying to compute the probability that after a perfect shuffling of a deck of memory cards (n pairs) none of the pairs end up with the two members next to each other.
I get into a messy ...

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**0**answers

143 views

### Inverse moment of the number of inversions of a permutation

Let $\pi$ be a permutation of $\{1,2,...,n\}$. A pair of elements ($\pi_i$,$\pi_j$) is called an inversion if $i$ $>$ $j$ and $\pi_i$ $<$ $\pi_j$. The total number of inversions in $\pi$ is ...

**16**

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**2**answers

385 views

### Shortest supersequence of all permutations of $n$ elements

Given an alphabet with $n$ characters, what is the shortest sequence that contains all $n!$ permutations as subsequences?
A subsequence can be obtained from a sequence by deleting any characters, ...

**7**

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**1**answer

146 views

### Distribution of entries of a doubly-sorted random matrix

Take an $n \times n$ random matrix whose entries are i.i.d. with uniform distribution in $[0,1]$. Look at the sums of the elements of each row and then permute the rows so that these sums form an ...

**10**

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**1**answer

299 views

### Explicit algorithm for composing permutations in factorial notation

Given two permutations p1 and p2 in factorial notation, is there an algorithm which computes their composition directly, i.e. without translating to a different notation (like Cauchy's 2-line ...

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**2**answers

193 views

### Combinatorial design for minimization problem over binary strings

Suppose the cost of a binary string $B$ of length $k$ is the number of $1$s that occur before the last $0$. For example, $1110$ has cost 3 while $0111$ has cost 0. Now suppose you can choose $k$ ...

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**1**answer

242 views

### Primitive, non-2-transitive groups with very large orbitals?

Let $G$ be a transitive permutation group on a set $X$ with $n$ elements. Assume that $G$ is primitive, i.e., $G$ preserves no non-trivial partition of $X$. Assume as well that $G$ is not ...

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**6**answers

657 views

### Why is the right permutohedron order (aka weak order) on $S_n$ a lattice?

This is one of those things I never expected to be hard until I tried to prove it. Why is the right permutohedron order (a.k.a. weak Bruhat order, a.k.a. weak order -- not to be confused with the ...

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**1**answer

108 views

### Cycles of Permutation Related to Rectangular Matrix Transposition

let the entries of a rectangular matrix $A\in\mathbb{C}^{m\times n}; m,n\in\mathbb{N}$ be stored in row-order in a linear vector $v$, i.e. $A_{i,j}=v_{i*m+j}$
Question:
How can the first element ...

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**3**answers

622 views

### How do most people write permutations?

I'd like to know how people prefer to write permutations, or elements of the symmetric group $S_n$ for $n\ge0$.
The most natural way to define a permutation in $S_n$ is as a bijection on the set ...

**0**

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**1**answer

192 views

### Permutations of letters under some conditions

Let $W(p,q,r,s)$ be the number of permutations of the letters which satisfy the following conditions :
Condition 1 : The letters are consist of $P,Q,R,S$.
Condition 2 : The number of letter ...

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**1**answer

183 views

### Effective Camille Jordan

It is a well-known (and frequently used) theorem of C. Jordan that a proper subgroup $H$ of a finite group $G$ can not intersect every conjugacy class. This is used most frequently for $G=S_n.$ Now, ...

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**1**answer

225 views

### Unimodality of length of longest increasing subsequence

For $w \in S_n$, the symmetric group on $n$ letters, let $\mathrm{is}(w)$ denote the length of the longest increasing subsequence of $w$. Define, $g_n(p) := |\{w \in S_n \colon \mathrm{is}(w) = p\}|$. ...

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**1**answer

283 views

### Combinatorial Technique Needed

The following problem is likely too special for MO.
However I have no clue how to deal with it, so I'll just try. Nevertheless
it is a combinatorial problem and a discussion about general methods
in ...

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**1**answer

79 views

### Probability of seeing m nonzero bits in off any d consecutive bits in a circle of n bits

Suppose n bits are arranged circularly with given condition that random k of them are 1 and rest 0, and all possible d consecutive bits (total n possibilities) are looked at, what is the probability ...

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**2**answers

110 views

### Draws from multiple non-disjoint urns

Let $C = \{1,...,n\}$ be a set of $n$ colors. Let $S_1,...,S_k$ be non-empty subsets of $C$, that is, $S_i \subseteq C$ for all $i \in \{1,...,k\}$. It is helpful to think of the $S_i$ as urns with ...

**9**

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**4**answers

874 views

### Number of Permutations?

Edit: This is a modest rephrasing of the question as originally stated below the fold: for $n \geq 3$, let $\sigma \in S_n$ be a fixed-point-free permutation. How many fixed-point-free permutations ...

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**2**answers

599 views

### Laws of Iterated Logarithm for Random Matrices and Random Permutation

The law of iterated logarithm asserts that if $x_1,x_2,\dots$ are i.i.d $\cal N(0,1)$ random variables and $S_n=x_1+x_2+\cdots+x_n$, then
$$\limsup_{n \to \infty} S_n/\sqrt {n \log \log n} = \sqrt 2, ...

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**1**answer

113 views

### GOE Version of Longest Increasing Subsequence

Let $S_n$ be the symmetric group equipped with uniform measure. For any $\pi\in S_n$, let $L_n=L_n(\pi)$ denote the longest increasing subsequence. A celebrated result of Baik, Deift and Johansson ...

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**1**answer

1k views

### How hard is reconstructing a permutation from its differences sequence?

My interest in combinatorially motivated computational problems led me to search for simple problems that turn out to be computationally hard. In this pursuit, I came up with a problem which I hope is ...

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**2**answers

2k views

### A question on maps from $\mathbb{Z}/p\mathbb{Z}$ to itself

Let $p\geq 3$ be a prime number, and let $u:\mathbb{Z}/p\mathbb{Z}\to \mathbb{Z}/p\mathbb{Z}$ be a map such that, for all $l\in \mathbb{Z}/p\mathbb{Z}$,$l\neq 0$, the map $k\mapsto u(k+l)-u(k)$ is a ...

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**1**answer

310 views

### Non-enumerative proof that there are many simple permutations?

Terence Tao asked for a non-enumerative proof that a positive proportion of permutations are derangements and got a great answer. Inspired by this, I'd like to ask about another family of ...

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**2**answers

357 views

### The proportion between permutations and derangements.

Denote the number of derangements by $D_N$. It's known that $D_N/N! \rightarrow 1/e$. Therefore $N!/e$ is an approximation for $D_N$.
I'm trying to bound the difference between this approximation and ...

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**0**answers

183 views

### A duality on partial permutations

A partial permutation matrix $\pi$ is one with at most one 1 in any row and column (the rest 0s). Given one, we can cross out to the East and South (but not Southeast) of each 1. Some boxes get ...

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**1**answer

351 views

### The distribution of cycle length in random derangement

It is known that for a fixed x $\in \{0,1,...,N-1\}$, the length of the cycle of x in a random permutation in $S_N$ distributes uniformly in
$\{1, . . . ,N\}$.
My question is regarding the length of ...

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**0**answers

98 views

### Notation for substructure, especially for permutations?

Is there a standard notation that expresses substructure?
The specific case that I care about is the following:
Suppose $\sigma,\tau$ are permutations such that $$\sigma(x)\not=x\implies ...

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**1**answer

263 views

### Permutations of prescribed cycle types that multiply to the identity

Suppose that $\lambda_1,\lambda_2,\lambda_3$ are partititions of $n$. When do there exist permutations $\sigma_1,\sigma_2,\sigma_3 \in S_n$ such that
(1) $\sigma_1\sigma_2\sigma_3$ is the identity;
...

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**0**answers

121 views

### Citation for subset complement result

Let $S=\lbrace s_1,\ldots,s_n \rbrace \subset \lbrace1,\ldots,2n\rbrace$. Consider two operations on $S$: the complement $C(S)=\lbrace 1,\ldots,2n \rbrace \setminus S$ and a reflection* ...

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**1**answer

519 views

### 3D generalizations of permutations, RSK correspondence, contingency tables, etc.

I want to gather facts and questions related to 3D generalizations
of permutations, RSK correspondence, contingency tables,
etc. One reason I am interested in this is because it is potentially
related ...

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**2**answers

377 views

### Distribution of distances in permutations

Let's consider all possible permutations of N numbers. Suppose for each permutation we calculate the sum of absolute differences between consecutive elements. Thus, for (1,2,3) one would have ...

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**1**answer

151 views

### Statistics on Lehmer codes

I am looking at words $\alpha_1 \ldots \alpha_n$, where $\alpha_j \in \{ 1, \ldots, j \}$. Thinking of $\alpha_j$ as a height, these words can be interpreted as left-to-right paths on the positive ...

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**1**answer

575 views

### Number of Permutations with k-inversions and with a single clamped value

This question is cross-posted from math.stackexchange because it might be too technical.
Let $S_n$ be the symmetric group. Recall that the number of inversions of a permutation $\sigma\in S_n$ is the ...

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**2**answers

272 views

### Maximum distance within a subset of permutations

I'm modelling a scheduler that accepts a sequence of requests and outputs a sequence of responses, one response per request. It can partially reorder requests, but only within a finite queue. ...

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**2**answers

216 views

### Is it an integer for all positive integer n ? [closed]

I am trying to figure out if the following expression
$$\frac{(n^2 - n)! }{ n! ((n-1)!)^n }$$
is an integer for all positive integer $n.$
I tried the induction, but induction case is running ...

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**2**answers

191 views

### Graph of $S_n$ with respect to transposition

Consider the graph $G_n$, with $V(G_n) = S_n$ (the set of permutations of a set of size $n$) and having an edge $\sigma\sigma'$ iif $\sigma'$ can be obtained from $\sigma$ by applying a transposition. ...

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**2**answers

448 views

### Permuting Racked Pool Balls with a Single Break

Given reasonable physical assumptions (on friction, collisions, etc.), would it be possible to "break" in a pool game such that when all the balls come to rest, the only difference is that the racked ...

**7**

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**4**answers

585 views

### Properties of permutations with unknown pattern avoidance descriptions

Background
Many properties of permutations can be stated in terms of classical patterns.
For example:
a permutation is stack-sortable if and only if it avoids 231 (Knuth 1975)
a permutation ...