[newbie] algsys: tried and failed to reduce system to a polynomial in one variable; give up.



I'm trying to determine 3 partial derivatives and figure out for which 
values they become zero, unfortunately this leads to the error mentioned in the 
subject:
p:tau11*tau22;
sigma:tau11+tau12+tau22;
A:sqrt(p^2+2*p*sigma)-p;
hjmax(p,sigma,A):=sqrt((1+A/p)/((1-A/sigma^2)^2+A/p));
subst([A=sqrt(p^2+2*p*sigma)-p,sigma=tau11+tau12+tau22,p=tau11*tau22],hjmax(
A,sigma,p));
eqs:[diff(%,tau11),diff(%,tau12),diff(%,tau22)];
unk:[tau11,tau12,tau22];
solve(eqs,unk);

can anyone here help me further?

thanks in advance
hugo