Grigory Perelman a no-show for his Fields Medal.Four mathematicians were today due to collect gold medals and glory in Madrid, Spain, having been declared winners of the 2006 Fields Medals — referred to as the 'Nobel prizes' of mathematics. But only three turned up. Grigory Perelman, a reclusive Russian mathematician who was widely expected to be one of this year's winners (see 'Maths 'Nobel' rumoured for Russian recluse'), was indeed honoured at the opening ceremony of the International Congress of Mathematicians. But after a round of applause, president of the International Mathematical Union John Ball said "I regret that Dr...