Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Some mathematical constants notably seem absent from the set of periods; in particular, it is not expected that Euler's number e and Euler-Mascheroni constant γ belong to mathcal{P}. The periods can be extended to exponential periods by permitting the product of an algebraic function and the exponential function of an algebraic function as an integrand. This extension includes all algebraic powers of e, the gamma function of rational arguments, and values of Bessel functions. If Euler's constant is added as a new period, then according to Kontsevich and Zagier "all classical constants are periods in the appropriate sense".