Without people noticing, modern life has been taken over. Algorithms run everything, from search engines on the internet, to satnavs and credit-card security - they even help people travel the world, find love and save lives. Here, mathematician Marcus du Sautoy demystifies this hidden world by showing some of the most essential algorithms. He reveals where these 2,000-year-old problem-solvers came from, showing how they work, what they have achieved and how they are now so advanced they can even program themselves