function x_A = xminusa(A)

% XMINUSA  Matrix of polynomials xI - A.
%	   X_A = XMINUSA(A) returns the matrix of polynomials xI - A
%	   where A is an n-by-n matrix of integers.
%	   X_A is an (nn + 2)-by-2 matrix in which the first row is
%	   [n 0], the second row is [1 0] and the remaining rows are either 
%	   [1, -a] for the corresponding entry a of A or [0, -a], depending 
%	   on whether the row corresponds to a diagonal entry of A or not.

%	   Adapted from Harold M. Edwards, "Linear Algebra."
%	   Written by Serge Tchikanda 03/26/96.
%          Last revised by Serge Tchikanda 06/13/96.

[m,n] = size(A);

if n ~= m
   disp(' ')
   disp('   Matrix must be square.')
   return
end  

v = ((n+1)*(0:(n-1))) + 1;
x_A1 = zeros(n*n,1);
x_A1(v) = ones(n,1);
x_A2 = -A(:);
x_A = [n 0;1 0;x_A1 x_A2];