Compressibility factor from the RK equation - one solution  |POLVER05_1
f(z)=z^3-z^2-Q*z-r    #
P = 200   #
Pc=33.5    #
T= 631    #
Tc=126.2    #
Pr=P/Pc    #
Tr=T/Tc    #
Asqr=0.4278*Pr/(Tr^2.5)    #
B=0.0867*Pr/Tr    #
Q=B^2+B-Asqr    #
r=Asqr*B    #
p=(-3*Q-1)/3    #
q=(-27*r-9*Q-2)/27    #
R=(p/3)^3+(q/2)^2    #
V=if(R>0)then((-q/2+sqrt(R))^(1/3))else(0)    #
WW=if(R>0)then(-q/2-sqrt(R))else(0)    #
psi1 = if(R<0)then(arccos(sqrt((q^2/4)/(-p^3/27))))else(0)    #
W = if(R>0)then(sign(WW)*(abs(WW))^(1/3))else(0)    #
z1 = if(R<0)then(2*sqrt(-p/3)*cos((psi1/3)+2*3.1416*0/3)+1/3)else(0)    #
z2 = if(R<0)then(2*sqrt(-p/3)*cos((psi1/3)+2*3.1416*1/3)+1/3)else(0)    #
z3 = if(R<0)then(2*sqrt(-p/3)*cos((psi1/3)+2*3.1416*2/3)+1/3)else(0)    #
z0=if(R>0)then(V+W+1/3)else(0)    #
z(min)=-0.5, z(max)=1.2