Leonhard Euler was undoubtedly the most productive mathematician of all time; he wrote a total of 866 papers and books on various branches of pure and applied mathematics, physics and astronomy, geodesy and cartography, and even music. Not only was he prolific, but in every field he addressed, he introduced novel ideas and, through his contributions, even opened up new areas of mathematics. The scientific world became aware of his extraordinary talent when, in 1735, he solved the so-called Basel problem, which had previously defeated all mathematicians.