The CDF of the student t distribution can be represented by $$\frac{1}{2} \cdot \beta_{x^2/(x^2+v)}\left(\frac 1 2,\frac v 2\right); \qquad x\in[-\sqrt{v},0]$$ Where we have a t distribution with $v>0$ degrees of freedom and $\beta_z(a,b)$ is the incomplete beta function.

I'm interested in it's derivative w.r.t. $v$, which happens to contain the term $$_3\tilde{F}_2\left(\frac{v}{2},\frac{v}{2},-\frac{1}{2};\frac{v}{2}+1,\frac{v}{2}+1;1-\frac{x^2}{x^2+v}\right)$$

I was able to get to this point using wolfram's derivatives of the incomplete beta function. However, for this to be useful to me, I would like to express it in terms of functions I'm more familiar with such as the gamma, polygamma, and beta functions. How might this be done? Also, how do the $\tilde{F}$ functions differ from the $F$ functions, and what are the $\tilde{F}$ functions called? I couldn't find any information on them at all.


1 Answer 1


It turns out that the $\tilde{F}$ functions are called regularized hypergeometric functions where $$_p\tilde{F}_{q}\left(a_1,...,a_p;b_1,...,b_q,z\right)=\frac{ _{p}F_{q}\left(a_1,...,a_p;b_1,...,b_q,z\right)}{\Gamma(b_1)\Gamma(b_2)...\Gamma(b_q)} $$

Additionally, matlab's symbolic hypergeometric function cannot find a way to express my function above in terms of any other functions so I'm assuming its either not possible or involved enough not to be worth the time investment.


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