The Distinguished Lecture Series, hosted by the School of Computing and Data Science (CDS), brings leading scholars from around the world to share their cutting-edge research and insights in the fields of computer science, data science, artificial intelligence, and statistics.

We are honoured to welcome Professor Gilles Brassard, a professor of computer science at the Université de Montréal, to our Distinguished Lecture Series. Professor Brassard will give an inspiring talk titled ” On computable numbers, with an application to the Druckproblem”.

 

Speaker:

Professor Gilles Brassard, Professor of Computer Science, Université de Montréal

 

Date:

21 August 2025 (Thursday)

Time:

11:00 am – 12:00 nn

Venue:

CPD-3.04, 3/F, Run Run Shaw Tower, Centennial Campus, The University of Hong Kong

Abstract:
In the famous paper “On computable numbers, with an application to the Entscheidungsproblem” in which Alan Turing introduced what is now known as the Turing machine, he gave a definition of computable real numbers under which it turns out that multiplication by 3 is uncomputable. I call such numbers “printable”. Despite the fact that the multiplication by 3 of printable numbers is uncomputable, as opposed to the same operation on computable numbers, a real number is computable if and only if it is printable. The resolution of this apparent paradox is that no machine can transform the “computable” description of a real number to its “printable” description. This is easy to prove if we insist that the decimal expansion of numbers of the form n/10^k must end with an infinite sequence of zeros, but significantly more subtle if we allow the choice to end such expansions with an infinite sequence of nines. In the process, I shall touch upon Brouwer’s philosophical issue of constructive mathematics and how it should appeal to computer scientists.

 

« This is joint work with Sophie Berthelette and Xavier Coiteux-Roy, based on paper https://doi.org/10.1016/j.tcs.2024.114573 » 

 

Biography:

Professor of computer science at the Université de Montréal since 1979, Gilles Brassard laid the foundations of quantum cryptography at a time when nobody could have predicted that the quantum information revolution would usher in a multi-billion dollar industry, much less that the United Nations would proclaim 2025 to be the International Year of Quantum Science and Technology. He is also among the inventors of quantum teleportation, which is one of the most fundamental pillars of the theory of quantum information. Fellow of the Royal Society of London, and Officer of the Order of Canada and Ordre national du Québec, his many accolades include the Wolf Prize in Physics, the Micius Quantum Prize, the BBVA Foundation Frontiers of Knowledge Award in Basic Sciences and the Breakthrough Prize in Fundamental Physics. He has been granted honorary doctorates from ETH Zürich, the University of Ottawa and Università della Svizzera italiana in Lugano.

All are welcome to attend.