Andrew Hem By Jacob Aron DEAR Fellow Researchers, I am pleased to announce a proof that P is not equal to NP, which is attached in 10pt and 12pt fonts.” So began an email sent in August 2010 to a group of leading computer scientists by Vinay Deolalikar, a mathematician at Hewlett-Packard Labs in Palo Alto, California. It was an incendiary claim. Deolalikar was saying he had cracked the biggest problem in computer science, a question concerning the complexity and fundamental limits of computation. Answer the question “Is P equal to NP?” with a yes, and you could be looking at a world transformed, where planes and trains run on time, accurate electronic translations are routine, the molecular mysteries of life are revealed at the click of a mouse – and secure online shopping becomes fundamentally impossible. Answer it with a no, as Deolalikar claimed to have done, and it would suggest that some problems are too irreducibly complex for computers to solve exactly. One way or another, then, we would like an answer. But it has been frustratingly slow in coming. “It’s turned out to be an incredibly hard problem,” says Stephen Cook of the University of Toronto, Canada, the computer scientist who first formulated it in May 1971. “I never thought it was going to be easy, but now 40 years later it looks as difficult as ever.” The importance of P = NP? was emphasised in 2000, when the privately funded Clay Mathematics Institute in Cambridge, Massachusetts, named it as one of seven “Millennium Prize” problems,