function v = exer7f(t,y,Z,h6,state)
ylag = Z;
v = zeros(4,1);

h1 = 2;
h2 = 0.8;
h3 = 1e4;
h4 = 0.17;
h5 = 0.5;
h7 = 0.12;
h8 = 8;
if state == +1
   xi = 1;
else
   xi = (1 - y(4))*10/9;
end

v(1) = (h1 - h2*y(3))*y(1);
v(2) = xi*h3*ylag(3)*ylag(1) - h5*(y(2) - 1);
v(3) = h4*(y(2) - y(3)) - h8*y(3)*y(1);
v(4) = h6*y(1) - h7*y(4);