double invf( double dF, int nNumDof, int nDenDof )
Returns F-distribution cumulative distribution function
dP=invf(dF,nNumDof,nDenDof); // results in dP==0.95
For historical reasons, this F cumulative distribution function was misnamed; Now, we developed a new set of distribution functions, whose CDFs end up with *cdf(), such as chi2cdf(), PDFs with *pdf(), and Inverse CDFs with *inv().
This function performs the same functionality as fcdf(), and we recommend the latter.