This is a list of misnamed theorems in mathematics. It includes theorems (and lemmas, corollaries, conjectures, laws, and perhaps even the odd object) that are well known in mathematics, but which are not named for the originator. That is, these items on this list illustrate Stigler's law of eponymy (which is not, of course, due to Stephen Stigler, who credits Robert K Merton).
== Applied mathematics ==
Benford's law. This was first stated in 1881 by Simon Newcomb, and rediscovered in 1938 by Frank Benford. The first rigorous formulation and proof seems to be due to Ted Hill in 1988.; see also the contribution by Persi Diaconis. Bertrand's ballot theorem. This result concerning the probability that the winner of an election was ahead at each step of ballot counting was first published by W. A. Whitworth in 1878, but named after Joseph Louis François Bertrand who rediscovered it in 1887. A common proof uses André's reflection method, though the proof by Désiré André did not use any reflections.
Algebra
Burnside's lemma. This was stated and proved without attribution in Burnside's 1897 textbook, but it had previously been discussed by Augustin Cauchy, in 1845, and by Georg Frobenius in 1887. Cayley–Hamilton theorem. The theorem was first proved in the easy special case of 2×2 matrices by Cayley, and later for the case of 4×4 matrices by Hamilton. But it was only proved in general by Frobenius in 1878. Hölder's inequality. This inequality was first established by Leonard James Rogers, and published in 1888. Otto Hölder discovered it independently, and published it in 1889. Marden's theorem. This theorem relating the location of the zeros of a complex cubic polynomial to the zeros of its derivative was named by Dan Kalman after Kalman read it in a 1966 book by Morris Marden, who had first written about it in 1945. But, as Marden had himself written, its original proof was by Jörg Siebeck in 1864. Pólya enumeration theorem. This was proven in 1927 in a difficult paper by J. H. Redfield. Despite the prominence of the venue (the American Journal of Mathematics), the paper was overlooked. Eventually, the theorem was independently rediscovered in 1936 by George Pólya. Not until 1960 did Frank Harary unearth the much earlier paper by Redfield. See for historical and other information.
Analysis
Frobenius theorem. This fundamental theorem was stated and proved in 1840 by Feodor Deahna. Even though Frobenius cited Deahna's paper in his own 1875 paper, it became known after Frobenius, not Deahna. See for a historical review. L'Hôpital's rule. This rule first appeared in l'Hôpital's book L'Analyse des Infiniment Petits pour l'Intelligence des Lignes Courbes in 1696. The rule is believed to be the work of Johann Bernoulli since l'Hôpital, a nobleman, paid Bernoulli a retainer of 300 francs per year to keep him updated on developments in calculus and to solve problems he had. See L'Analyse des Infiniment Petits pour l'Intelligence des Lignes Courbes and reference therein. Maclaurin series. The Maclaurin series was named after Colin Maclaurin, a professor in Edinburgh, who published this special case of the Taylor series in 1742, but never claimed to have discovered it. Poincaré lemma. This was mentioned in 1886 by Henri Poincaré, but was first proved in a series of 1889 papers by the distinguished Italian mathematician Vito Volterra. Nonetheless, it has become known after Poincaré. See for the twisted history of this lemma. Stokes' theorem. It is named after Sir George Gabriel Stokes (1819–1903), although the first known statement of the theorem is by William Thomson (Lord Kelvin) and appears in a letter of his to Stokes. The theorem acquired its name from Stokes' habit of including it in the Cambridge prize examinations. In 1854 he asked his students to prove the theorem in an examination; it is not known if anyone was able to do so.