'Return the span of the simple roots
The root space is the vector space spanned by the simple roots, i.e. it
is a vector space with a distinguished basis, the simple roots.  This
method returns a string that represents the root space as the span of
the simple roots, alpha[1],...., alpha[n].
Examples
>>> from sympy.liealgebras.root_system import RootSystem
>>> c = RootSystem("A3")
>>> c.root_space()
\'alpha[1] + alpha[2] + alpha[3]\''

def root_space(self):

    n = self.cartan_type.rank()
    rs = '    +    '.join(((('alpha[' + str(i)) + ']') for i in range(1, (n + 1))))
    return rs
