function [r] = ptest3(a)


% Return r=1 if `a' is a P-matrix (r=0 otherwise). 


n = length(a);
if ~(a(1,1)>0), r = 0;
elseif n==1, r = 1;   
else
   b = a(2:n,2:n);
   d = a(2:n,1)/a(1,1);
   c = b - d*a(1,2:n);  
   r = ptest3(b) & ptest3(c);
end