The F (or Fisher Snedecor) cumulative distribution function
F = pf(x,df1,df2)
real
degrees of freedom (x,df1,df2 can be scalar or matrix with common size)
for each element of x, F=Prob((X/df1)/(Y/df2) < x) where X and Y are independent Chi-square random variables with one degree of freedom
The F (or Fisher-Snedecor) cumulative distribution function