Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In geometry, the Cesàro equation of a plane curve is an equation relating curvature ( ) to arc length (s). It may also be given as an equation relating the radius of curvature (R) to arc length. (These are equivalent because R = 1 / .) Two congruent curves will have the same Cesàro equation. It is named for Ernesto Cesàro. Determining the length of an irregular arc segment also called rectification of a curve was historically difficult. Although many methods were used for specific curves, the advent of calculus led to a general formula that provides closed-form solutions in some cases.