'The Cartan matrix for G_2
The Cartan matrix matrix for a Lie algebra is
generated by assigning an ordering to the simple
roots, (alpha[1], ...., alpha[l]).  Then the ijth
entry of the Cartan matrix is (<alpha[i],alpha[j]>).
Examples
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("G2")
>>> c.cartan_matrix()
Matrix([
[ 2, -1],
[-3,  2]])'

def cartan_matrix(self):

    m = Matrix(2, 2, [2, (-1), (-3), 2])
    return m
