 Many of the constants known to be periods are also given by integrals of transcendental functions.Kontsevich and Zagier note that there "seems to be no universal rule explaining why certain infinite sums or integrals of transcendental functions are periods".Kontsevich and Zagier conjectured that, if a period is given by two different integrals, then each integral can be transformed into the other using only the linearity of integrals, changes of variables, and the Newton–Leibniz formula

(or, more generally, the Stokes formula).