 Though the exact period formula_32 can be determined, for any finite amplitude formula_33 rad, by evaluating the corresponding complete elliptic integral formula_34, where formula_35, this is often avoided in applications because it is not possible to express this integral in a closed form in terms of elementary functions.This has made way for research on simple approximate formulae for the increase of the pendulum period with amplitude (useful in introductory physics labs, classical mechanics, electromagnetism, acoustics, electronics, superconductivity, etc.The approximate formulae found by different authors can be classified as follows