DF

df
The F density function (or Fisher-Snedecor density function)

         f = df(x,df1,df2)
	Input	x	real (scalar, vector or matrix)
		df1,df2	degrees of freedom

	Output	f	F density function at the value of x, of 
			(X/df1)/(Y/df2) where X and Y are independent 
			Chi^2 variables with one degree of freedom