  | ### 5.1.85 luinverse qcindex Gauss `Syntax:` `luinverse (` matrix_expression `)` `Type:` matrix `Syntax:` `luinverse (` matrix_expression`,` matrix_expression`,` matrix_expression `)` `Type:` matrix `Purpose:` Computes the inverse of a matrix A, if A is invertible. The matrix A must be given either directly, or by its LU-decomposition. In the latter case, three matrices P, L, and U are expected, in this order, which satisfy - P * A = L * U, - P is an (m x m) permutation matrix, i.e., its rows/columns form the standard basis of K^m, - L is an (m x m) matrix in lower triangular form with all diagonal entries equal to 1, and - U is an (m x m) matrix in upper row echelon form. Then, the inverse of A exists if and only if U is invertible, and one has ,since P is self-inverse. In the case of A being given directly, `luinverse` first computes its LU-decomposition, and then proceeds as in the case when P, L, and U are provided. `list L=luinverse(A);` fills the list L with either one entry = 0 (signaling that A is not invertible), or with the two entries .Thus, in either case the user may first check the condition `L[1]==1` to find out whether A is invertible. `Note:` The method will give a warning for any non-quadratic matrix A. `Example:`
        ring r=0,(x),dp;
        matrix A[3][3]=1,2,3,1,1,1,2,2,1;
        list L = luinverse(A);
        if (L[1] == 1)
        {
          print(L[2]);
          "----- next should be the (3 x 3)-unit matrix:";
          print(A*L[2]);
        }
      ==> -1,4, -1,
      ==> 1, -5,2,
      ==> 0, 2, -1
      ==> ----- next should be the (3 x 3)-unit matrix:
      ==> 1,0,0,
      ==> 0,1,0,
      ==> 0,0,1
See `system("rref",A);`. See ludecomp; lusolve; system.

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