## Determinants of Bohemian matrix families

The adjective “Bohemian” was used for the first time in a linear algebra context by Robert Corless and Steven Thornton to describe the eigenvalues of matrices whose entries are taken from a finite discrete set, usually of integers. The term is a partial acronym for “BOunded Height Matrix of Integers”, but the origin of the term was soon forgotten, and the expression “Bohemian matrix” is now widely accepted.

As Olga Taussky observed already in 1960, the study of matrices with integer elements is “very vast and very old”, with early work of Sylvester and Hadamard that dates back to the second half of the nineteenth century. These names are the first two in a long list of mathematicians that worked on what is now known as the “Hadamard conjecture”: for any positive integer $n$ multiple of 4, there exists an $n$ by $n$ matrix $H$, with entries $-1$ and $+1$, such that $HH^T = nI$.

If this is the best-known open problem surrounding Bohemian matrices, it is far from being the only one. During the 3-day workshop “Bohemian Matrices and Applications” that our group hosted in June last year, Steven Thornton released the Characteristic Polynomial Database, which collects the determinants and characteristic polynomials of billions of samples from certain families of structured as well as unstructured Bohemian matrices. All the available data led Steven to formulate a number of conjectures regarding the determinants of several families of Bohemian upper Hessenberg matrices.

Gian Maria Negri Porzio and I attended the workshop, and set ourselves the task of solving at least one of these open problems. In our recent preprint, we enumerate all the possible determinants of Bohemian upper Hessenberg matrices with ones on the subdiagonal. We consider also the special case of families with main diagonal fixed to zero, whose determinants turn out to be related to some generalizations of Fibonacci numbers. Many of the conjectures stated in the Characteristic Polynomial Database follow from our results.

## Highlights of Advances in Numerical Linear Algebra Conference

by Sven Hammarling, Nick Higham and Françoise Tisseur

The conference Advances in Numerical Linear Algebra: Celebrating the Centenary of the Birth of James H. Wilkinson, took place at the University of Manchester, May 29-30, 2019.  The purpose of the conference was to discuss recent developments and future challenges in numerical linear algebra and to celebrate the centenary of the birth of James H. Wilkinson FRS (1919-1986).

The 60 attendees heard reminiscences about Wilkinson and his work from several attendees who knew him.  Sven Hammarling opened the proceedings with personal reflections on Wilkinson. Margaret Wright discussed some of the treasures in lecture notes from courses that Wilkinson taught at Stanford University (1977-1982). We have recently added these notes to our Wilkinson web page. Nick Higham announced the availability of the Argonne tapes, which are videos of Wilkinson and Cleve Moler lecturing at an Eigensystem Workshop held at Argonne National Laboratory, Illinois, USA, in June 1973.

We were pleased to welcome two of Jim Wilkinson’s nephews, John Liebman and Danny Liebman, together with John’s wife Liz.

Attendees were able to wish speaker Cleve Moler a happy 80th birthday and share two birthday cakes with him.

Photos from the conference appear below. Click on a photo to enlarge it.

Slides of the talks are available at the conference web page. The first day’s lectures were professionally videoed and the videos are available on the NLA group’s YouTube channel: here is a link to the playlist.

Thank you to all the speakers and attendees for their participation.

We gratefully acknowledge sponsorship from the Royal Society, The Alan Turing Institute, The QJMAM Fund for Applied Mathematics, The Numerical Algorithm Group and National Physical Laboratory.