error in the following code

alpha = 0.087; lamdas =0; mu_0 =1.1; Tf =1793; q =3.6;

Tint= 573; % initial assumed value

%Tf= 1793; % melting point of the material

%mu_0= 1.1; % property of the material in consideration

%q= 3.6; % property of the material in consideration

mu = mu_0*(1-(Tint/Tf)^q);

lamda= atan(mu); % friction angle

phi= (pi/4)+ 0.5*(alpha-atan(mu));

syms phi alpha lamda lamdas x

p=(cos(phi-alpha)*sin(phi)*sin(x))-(tan(lamdas)*((cos(phi-alpha))^2)*cos(x))+((cos(alpha)- sin(phi-alpha)*sin(phi))*tan(lamda)*sin(x)*cos(x))+(tan(lamda)*tan(lamdas)*sin(phi- alpha)*cos(phi-alpha)*(cos(x))^2);

q=diff(p,x);

r=diff(q,x);

f= @(x) (p);

df= @(x) (q);

ddf= @(x) (r);

x1= 0.1;

acc= 0.001;

y1= f(x1);

y2= df(x1);

y3= ddf(x1);

while(((y1*y3)/(y2*y2))>1),

    x1=0.1;

    y1=     f(x1);

    y2=     df(x1);

    y3=     ddf(x1);

end;

x2=(x1-(y1/y2));

while (abs(x2-x1)>acc),

    x1=x2;

    y1=     f(x1);

    y2=     df(x1);

    %y3=     ddf(x1);

    x2=(x1-(y1/y2));

end;

x= fzero(@(x)(p),1);

for the above code, the error i am getting is as follows:

??? Error using ==> sym.sym>notimplemented at 2653 Function 'gt' is not implemented for MuPAD symbolic objects.

Error in ==> sym.sym>sym.gt at 801 notimplemented('gt');

Error in ==> diff_eta_c at 35 while(((y1*y3)/(y2*y2))>1),

thank you :-)