In mathematics, Clausen's formula, found by Thomas Clausen (1828), expresses the square of a Gaussian hypergeometric series as a generalized hypergeometric series. It states
![{\displaystyle \;_{2}F_{1}\left[{\begin{matrix}a&b\\a+b+1/2\end{matrix}};x\right]^{2}=\;_{3}F_{2}\left[{\begin{matrix}2a&2b&a+b\\a+b+1/2&2a+2b\end{matrix}};x\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1ef3f808aeaeb371d8111d862a7c02000a5e13e8)
In particular it gives conditions for a hypergeometric series to be positive. This can be used to prove several inequalities, such as the Askey–Gasper inequality used in the proof of de Branges's theorem.