dx1/dt = -bi*x1*x2 - x1*mu - x1*bw*x3 + x4*a + mu; dx2/dt = bi*x1*x2 - gam*x2 + x1*bw*x3 - mu*x2; dx3/dt = xi*x2 - xi*x3; dx4/dt = gam*x2 - x4*mu - x4*a; y1 = k*x2