Tag Archives: Ramanujan graphs

Ramanujan+100

I have just spent the last four days in Kochi, Kerala, at the International Conference on Number Theory and Discrete Mathematics, commemorating Srinivasa Ramanujan, the great Indian mathematician, on the 100th anniversary of his far-too-early death. The conference had perhaps … Continue reading

Posted in doing mathematics, events, open problems | Tagged , , , , | 4 Comments

Peter Sarnak’s Hardy Lecture

Yesterday, Peter Sarnak gave the London Mathematical Society’s 2020 Hardy Lecture (remotely). He talked about gaps in the spectra of connected cubic graphs. It was a talk properly described as a tour de force, applying to the problem ideas from … Continue reading

Posted in events, exposition | Tagged , , , , , | Leave a comment

Asymptotic group theory, 3

A Ramanujan graph is a connected finite graph of valency k whose eigenvalues (apart from k and −k) all have modulus at most 2√(k−1). This interval is the spectrum of the infinite k-valent tree T (regarded as an operator on … Continue reading

Posted in events, exposition | Tagged , , , , , , , , , | Leave a comment