defined by Christiaan Huygens: Huygens, Christian (1673). "Horologium Oscillatorium"(PDF). 17centurymaths. 17thcenturymaths.com. Retrieved 2009-03-01., Part 4, Definition 3, translated July 2007 by Ian Bruce
Gil, Salvador; Legarreta, Andrés E.; Di Gregorio, Daniel E. (September 2008). "Measuring anharmonicity in a large amplitude pendulum". American Journal of Physics. 76 (9): 843–847. Bibcode:2008AmJPh..76..843G. doi:10.1119/1.2908184. ISSN0002-9505.
gsu.edu
hyperphysics.phy-astr.gsu.edu
Nave, Carl R. (2006). "Simple pendulum". Hyperphysics. Georgia State Univ. Retrieved 2008-12-10.
Gil, Salvador; Legarreta, Andrés E.; Di Gregorio, Daniel E. (September 2008). "Measuring anharmonicity in a large amplitude pendulum". American Journal of Physics. 76 (9): 843–847. Bibcode:2008AmJPh..76..843G. doi:10.1119/1.2908184. ISSN0002-9505.
Gil, Salvador; Legarreta, Andrés E.; Di Gregorio, Daniel E. (September 2008). "Measuring anharmonicity in a large amplitude pendulum". American Journal of Physics. 76 (9): 843–847. Bibcode:2008AmJPh..76..843G. doi:10.1119/1.2908184. ISSN0002-9505.