R.PF(x,n1,n2,lower_tail,log_p)
x: observation
n1: the first number of degrees of freedom of the distribution
n2: the second number of degrees of freedom of the distribution
lower_tail: if true (the default), the lower tail of the distribution is considered
log_p: if true, log of the probability is used
This function returns the cumulative distribution function of the F distribution.