Why Error in while loop?

x10=2;
x20=-2;
x30=1;
e1=0.00001;
iter=10;
m=2;

z = sym('2*x1 * x1 + x2*x2 + 3*x3*x3');
syms x1 x2 x3 a;

gradz = -[diff(z,x1); diff(z,x2); diff(z,x3)]; % returns a column vector

p=subs(gradz, [x1 x2 x3], [x10 x20 x30]); % evaluate at (0.25, 0.75)

x1a0=x10+a*p(1);
x2a0=x20+a*p(2);
x3a0=x30+a*p(3);

za0=subs(z, [x1 x2 x3], [x1a0 x2a0 x3a0]);
dza0=diff(za0,a);
k=solve(dza0);
x11=vpa(x10+k*p(1),8);
x21=vpa(x20+k*p(2),8);
x31=vpa(x30+k*p(3),8);

cltr1=((x11-x10)/x10);

while ((m<iter) && (cltr1>e1))
    m=m+1;

p=subs(gradz, [x1 x2 x3], [x11 x21 x31]);

x1a=x11+p(1)*a;
x2a=x21+p(2)*a;
x3a=x31+p(3)*a;

za1=subs(z, [x1 x2 x3], [x1a x2a x3a]);
dza1=diff(za1,a);
w=solve(dza1);

x12=x11+p(1)*w;
x22=x21+p(2)*w;
x32=x31+p(3)*w;

x1k=x11;
x2k=x21;
x3k=x31;

x11=x12;
x21=x22;
x31=x32;

end

display(x12) 
display(x22)
display(x32)

There is error in executing while command. Error is as follow: ??? Error using ==> sym.sym>notimplemented at 2514 Function 'gt' is not implemented for MuPAD symbolic objects.

Error in ==> sym.sym>sym.gt at 792 notimplemented('gt');

Error in ==> CAUCHY_1 at 28 while ((m<iter) && (cltr1>0.001))

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/69281-error-in-using-while-loop