Elmer Gethin Rees, CBE FRSE (19 November 1941 – 4 October 2019[1][2]) was a Welsh mathematician with publications in areas ranging from topology, differential geometry, algebraic geometry, linear algebra and Morse theory to robotics.

He held the post of Director of the Heilbronn Institute for Mathematical Research, a partnership between the University of Bristol and the British signals intelligence agency GCHQ, from its creation in 2005 until 2009.

He studied at St Catharine's College, Cambridge gaining a BA before moving on to the University of Warwick, where he completed his PhD in 1967.

Rees was appointed Commander of the Order of the British Empire (CBE) in the 2009 Birthday Honours.

[3] While at the universities of Oxford and Edinburgh, he supervised at least 15 PhD students,[4][5] including Anthony Bahri, John D. S. Jones, Gregory Lupton, Jacob Mostovoy, Simon Willerton and Richard Hepworth.