# Characteristic Polynomial Database

The **characteristic polynomial database** contains characteristic polynomials,
minimal polynomials and properties for a variety of families of Bohemian matrices. Currently the database contains **1,762,728,065** characteristic polynomials from **2,366,960,967,336** matrices.

## Bohemian Families

### Unstructured

Matrices with no structure. For example, $5 \times 5$ matrices with entries from the set $\{-1, 0, +1\}$.

### Upper Hessenberg

Upper Hessenberg matrices with subdiagonal entries fixed at 1.

## Data Files

### Polynomial Files

All characteristic polynomial and minimal polynomial files are CSV files with
no header. They have been provided in uncompressed format, as well as
compressed as zip files and tar.gz files. The characteristic polynomial files
follow the naming convention `CharPolys_nxn.csv`

where `n`

is the dimension of
the matrices in the family they are computed from. Similarly, the minimal
polynomial files follow the naming convention `MinPolys_nxn.csv`

where `n`

is
the dimension of the matrices in the family they are computed from. Each row in the CSV files is of the form:

`count, c_{n}, c_{n-1}, ..., c_0`

where `count`

is the number of times the characteristic polynomial appears in
the family. `c_{n}`

to `c_0`

are the coefficients of the characteristic
polynomial beginning with the degree n coefficient down to the constant
coefficient.

A Maple function `read_poly_data`

is available here for reading the polynomials (and optionally their counts) into Maple. See the header in the file for details on its usage. Polynomials can be read into Maple as follows:

```
# Read polynomials into an array, counts are discarded.
char_polys := read_poly_data("CharPolys_4x4.csv", x):
# Read polynomials and their counts into arrays.
char_polys, char_poly_counts := read_poly_data("CharPolys_4x4.csv", x, keepCount=true):
```

## Property Definitions

### Number of Matrices

The number of matrices in the family.

### Number of Characteristic Polynomials

The number of distinct characteristic polynomials in the family.

### Number of Minimal Polynomials

The number of distinct minimal polynomials of the matrices in the family.

### Number of Non-Derogatory Matrices

The number of matrices in the family where their minimal polynomial and characteristic polynomial coincide (up to a factor of $\pm1$), see

https://www.encyclopediaofmath.org/index.php/Non-derogatory_matrix.

### Maximum Characteristic Height

The characteristic height of a matrix is the maximum absolute coefficient of its characteristic polynomial when written in the monomial basis. The maximum characteristic height over a family of Bohemian matrices is the maximum of the characteristic heights of the matrices in the family.

### Number of Distinct Eigenvalues

The cardinality of the set of all eigenvalues from all matrices in a Bohemian family.

### Number of Distinct Real Eigenvalues

The cardinality of the subset of all eigenvalues from all matrices in a Bohemian family that have zero complex part.

### Number of Distinct Purely Complex Eigenvalues

The cardinality of the subset of all eigenvalues from all matrices in a Bohemian family that have non-zero complex part.

### Number of Distinct Jordan Canonical Forms

The cardinality of the set of Jordan canonical forms from all matrices in a Bohemian family up to permutations of the Jordan blocks. More precisely, this is the cardinality of the set of all Frobenius forms from all matrices in a Bohemian family. Or equivalently, the cardinality of the subset of a family of Bohemian matrices such that no two matrices in the subset are similar.

### Number of Singular Matrices

Number of matrices that are singular (i.e. $\det(A) = 0$).

### Number of Non-Singular Matrices

Number of matrices that are non-singular (i.e. $\det(A) \ne 0$).

### Number of Rank n Matrices

Number of matrices in the Bohemian family with a rank of n.

### Number of Distinct Determinants

Number of unique determinants in the family of Bohemian matrices.

### Maximum Determinant

Maximum absolute determinant over all matrices in the family.

### Number of Unimodular Matrices

Number of unimodular matrices, i.e. matrices with determinant of $+1$ or $-1$.

### Number of Normal Matrices

Number of matrices in the family such that $AA^T - A^TA = 0$.

### Number of Rhapsodic Matrices

Number of matrices in the Bohemian family such that its inverse also belongs to the same Bohemian family.

### Number of Nilpotent Matrices

Number of matrices such that there exists a positive integer $k$ where $A^k = 0$.

### Number of Totally Unimodular Matrices

Number of matrices in the Bohemian family such that every square non-singular submatrix is unimodular.

### Number of Type I Stable Matrices

Number of matrices in the Bohemian family where the real part of all eigenvalues is strictly negative ($\textrm{Re}(\lambda) < 0$). These are the matrices $A$ where the differential equation $\dot{y} = Ay$ will ultimately decay as $t \to \infty$.

### Number of Type II Stable Matrices

Number of matrices in the Bohemian family where all eigenvalues are strictly within the unit circle ($|\lambda| < 1$). These are the matrices where the recursion $y_{n+1} = A y_n$ will ultimately decay to 0 as $n \to \infty$.

## Conjectures

While computing properties for the families of matrices available in the database, we came across many cases where a property appears to match a sequence on the OEIS but we were unable to find a proof. Below we list sequences that appear to match, for which we have no proof of their validity. If you prove any of the conjectures below, or know of a reference with their proof, please send an email to Steven Thornton at sthornt7@uwo.ca, or Rob Corless at rcorless@uwo.ca with details. Full credit will be given.

#### Conjecture 1

**Conjecture:**The number of nilpotent $n \times n$ matrices with entries from the set $\{0, +1\}$ is given by the sequence A003024.**Status:**True**Reference:**Cvetković, D., Doob, M., & Sachs, H. (1980). Spectra of Graphs-Theory and Application. Third ed., Barth, Heidelber, 1995.**Proof:**[p. 81] show that a digraph G contains no cycle if and only if all eigenvalues of the adjacency matrix are 0, which is the explicit bijection between nilpotent $n×n$ matrices and DAGs.**Proof Provided By:**Jianxiang Chen

#### Conjecture 2

**Conjecture:**The maximal characteristic height of $n \times n$ matrices with entries from the set $\{0, +1\}$ is given by the sequence A082914.**Status:**False**Proof:**According to the conjecture, the largest characteristic height of a $20 \times 20$ $\{0,1\}$ matrix should be 9754214400. But observe that the $j$-th coefficient of the characteristic polynomial is an alternate sum of all the determinants of the $(n − j) \times (n − j)$ diagonal minors, and the determinant of a $\{0,1\}$ matrix is bounded by A003432, the coefficient is bounded by $C(n,j)$A003432$(n-j)$. The values of $C(n,j)$A003432$(n-j)$ for a $20\times20$ matrix are: 390625000, 648273920, 1270087680, 1587609600, 2032140288, 988961400, 734657040, 459160650, 244885680, 59121920, 24186240, 7054320, 2480640, 348840, 77520, 14535, 2280, 190, 20, which are all smaller than 9754214400. Therefore, the conjecture is false.**Proof Provided By:**Jianxiang Chen

#### Conjecture 3

**Conjecture:**The number of nilpotent $n \times n$ matrices with entries from the set $\{0, +1\}$ and diagonal entries fixed at 0 is given by the sequence A003024.**Status:**True**Reference:**Cvetković, D., Doob, M., & Sachs, H. (1980). Spectra of Graphs-Theory and Application. Third ed., Barth, Heidelber, 1995.**Proof:**[p. 81] show that a digraph G contains no cycle if and only if all eigenvalues of the adjacency matrix are 0, which is the explicit bijection between nilpotent $n×n$ matrices and DAGs.**Proof Provided By:**Jianxiang Chen

#### Conjecture 4

**Conjecture:**The maximal absolute determinant of $n \times n$ matrices with entries from the set $\{-1, 0, +1\}$ is given by the sequence A003433.**Status:**True**Proof:****Lemma.**For square matrices $A$, $B$, $C$ with $2A_{m,n}=B_{m,n}+C_{m,n}$ and all other elements equal, their determinants satisfy $2|A|=|B|+|C|$.**Proof.**By Laplace expansion on the $m$th row or $n$th column. So for every $\{-1,0,1\}$ matrix with some element $A_{m,n}=0$, we can construct two matrices $B$ and $C$ with $B_{m,n}=1$ and $C_{m,n}=-1$ with all other elements equal. Then at least one of $|B|$ and $|C|$ is not smaller than $|A|$. By repeating the procedure we can eliminate all zeros of $A$, arriving at a $\{-1,1\}$ matrix with maximum determinant, which is the definition of A003433.**Proof Provided By:**Jianxiang Chen

#### Conjecture 5

**Conjecture:**The number of nilpotent $n \times n$ matrices with entries from the set $\{-1, 0, +1\}$ and diagonal entries fixed at 0 is given by the sequence A085506.**Status:**True**Reference:**McKay, B. D., Oggier, F. E., Royle, G. F., Sloane, N. J. A., Wanless, I. M., & Wilf, H. S. (2004). Acyclic digraphs and eigenvalues of (0, 1)-matrices.*Journal of Integer Sequences*, 7(2), 3.**Proof:**Follows from proof on [p. 2].**Proof Provided By:**Jianxiang Chen

#### Conjecture 6

**Conjecture:**The number of nilpotent $n \times n$ matrices with entries from the set $\{0, +1, +2\}$ is given by the sequence A188457.**Status:**Open

#### Conjecture 7

**Conjecture:**The maximum absolute determinant of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{0, +1\}$ and subdiagonal entries fixed at 1 is given by the Fibonacci sequence A000045.**Status:**True**Reference:**Ching, L. (1993). The maximum determinant of an $n \times n$ lower Hessenberg (0, 1) matrix.*Linear algebra and its applications, 183*, 147-153.**Proof Provided By:**Nick Higham

#### Conjecture 8

**Conjecture:**The maximum absolute determinant of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{0, +1, +2\}$ and subdiagonal entries fixed at 1 is given by sequence A052542.**Status:**Open

#### Conjecture 9

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{0, 1\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by sequence A212264.**Reference:**Du, Z., da Fonseca, C. M., Xu, Y., & Ye, J. (2021). Disproving a conjecture of Thornton on Bohemian matrices.*Open Mathematics, 19*(1), 505-514.**Status:**False

#### Conjecture 10

**Conjecture:**All upper-Hessenberg matrices with subdiagonal entries fixed at 1 are non-derogatory.**Status:**True**Reference:**Chan, E., Corless, R. M., Gonzalez-Vega, L., Sendra, J. R., Sendra, J., & Thornton, S. (2018). Bohemian Upper Hessenberg Matrices.*arXiv preprint arXiv:1809.10653*,**Proof:**Proposition 5.3.**Proof Provided By:**Steven E. Thornton

#### Conjecture 11

**Conjecture:**The maximum characteristic height of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{0, +1, +2\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by sequence A058764.**Status:**Open

#### Conjecture 12

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by sequence A001611.**Status:**Open

#### Conjecture 13

**Conjecture:**The maximum absolute determinant of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by the Fibonacci sequence A000045.**Status:**Open

#### Conjecture 14

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0, +1\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by sequence A001588.**Status:**Open

#### Conjecture 15

**Conjecture:**The maximum absolute determinant of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0, +1\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by the Fibonacci sequence A000045.**Status:**Open

#### Conjecture 16

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, +1\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by sequence A001611.**Status:**Open

#### Conjecture 17

**Conjecture:**The maximum absolute determinant of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, +1\}$, subdiagonal entries fixed at 1, and diagonal entries fixed at 0 is given by the Fibonacci sequence A000045.**Status:**Open

#### Conjecture 18

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0\}$ and subdiagonal entries fixed at 1 is given by sequence A000051.**Status:**Open

#### Conjecture 19

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, 0, +1\}$ and subdiagonal entries fixed at 1 is given by sequence A000051.**Status:**Open

#### Conjecture 20

**Conjecture:**The number of distinct determinants of an $n \times n$ upper-Hessenberg matrix with entries from the set $\{-1, +1\}$ and subdiagonal entries fixed at 1 is given by sequence A000051.**Status:**Open

## Referencing the CPDB

If the database helped your work and you wish to reference it, the following citation format should be used:

S. E. Thornton, *The Characteristic Polynomial Database*, published electronically at http://bohemianmatrices.com/cpdb, [date]

### Bibtex

```
@online{CPDB,
author = {Steven E. Thornton},
title = {The Characteristic Polynomial Database},
url = {http://bohemianmatrices.com/cpdb},
urldate = {[date]}
}
```

or

```
@misc{CPDB,
author = {Steven E. Thornton},
title = {The Characteristic Polynomial Database},
howpublished = {Available at \url{http://bohemianmatrices.com/cpdb} ([date])}
}
```