English mathematician Alan Turing (1912-1954) is the author of the 1936 paper On Computable Numbers, with an Application to the Entscheidungsproblem that introduced the imaginary computer called the Turing Machine for understanding the nature and limitations of computing. His famous 1950 article Computing Machinery and Intelligence introduced the Turing Test for gauging artificial intelligence. American writer Charles Petzold (1953-) is the author of the acclaimed 1999 book Code: The Hidden Language of Computer Hardware and Software, a unique exploration into the digital technologies of computers. He is also the author of hundreds of articles about computer programming, as well as several books on writing programs that run under Microsoft Windows. His Web site is www.charlespetzold.com.