F15b:= 1+0.75*x+0.42*y-1.00*z-5.50*x^2-1.42*x*y-1.58*y^2-6.63*x*z-0.79*y*z-0.75*z^2+4.75*x^3+3.75*x^2*y
       +2.25*x*y^2+0.58*y^3-1.50*x^2*z-0.50*x*y*z+2.50*y^2*z+1.75*x*z^2
       +2.08*y*z^2+2.00*z^3+2.00*x^4-2.00*x^3*y-1.00*x*y^3+9.50*x^3*z
       -3.00*x^2*y*z+3.00*x*y^2*z-1.00*y^3*z-3.00*x^2*z^2-1.00*x*y*z^2
       -1.00*y^2*z^2+3.00*x*z^3-1.00*y*z^3-1.00*z^4
       -3.00*x^5-1.00*x^3*y^2-1.00*x^3*z^2;