 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, further, Euler's constant γ is added as a new period, then according to Kontsevich and Zagier "all classical constants are periods in the appropriate sense".