4.14.7 Linear algebra

The matrix class can be used with indices to do some simple linear algebra (linear combinations and products of vectors and matrices, traces and scalar products):

{
    idx i(symbol("i"), 2), j(symbol("j"), 2);
    symbol x("x"), y("y");

    // A is a 2x2 matrix, X is a 2x1 vector
    matrix A(2, 2), X(2, 1);
    A = 1, 2,
        3, 4;
    X = x, y;

    cout << indexed(A, i, i) << endl;
     // -> 5

    ex e = indexed(A, i, j) * indexed(X, j);
    cout << e.simplify_indexed() << endl;
     // -> [[2*y+x],[4*y+3*x]].i

    e = indexed(A, i, j) * indexed(X, i) + indexed(X, j) * 2;
    cout << e.simplify_indexed() << endl;
     // -> [[3*y+3*x,6*y+2*x]].j
}

You can of course obtain the same results with the matrix::add(), matrix::mul() and matrix::trace() methods (see section Matrices) but with indices you don't have to worry about transposing matrices.

Matrix indices always start at 0 and their dimension must match the number of rows/columns of the matrix. Matrices with one row or one column are vectors and can have one or two indices (it doesn't matter whether it's a row or a column vector). Other matrices must have two indices.

You should be careful when using indices with variance on matrices. GiNaC doesn't look at the variance and doesn't know that ‘F~mu~nu’ and ‘F.mu.nu’ are different matrices. In this case you should use only one form for ‘F’ and explicitly multiply it with a matrix representation of the metric tensor.