'Every lie algebra has a unique root system.
Given a root system Q, there is a subset of the
roots such that an element of Q is called a
simple root if it cannot be written as the sum
of two elements in Q.  If we let D denote the
set of simple roots, then it is clear that every
element of Q can be written as a linear combination
of elements of D with all coefficients non-negative.
In A_n the ith simple root is the root which has a 1
in the ith position, a -1 in the (i+1)th position,
and zeroes elsewhere.
This method returns the ith simple root for the A series.
Examples
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("A4")
>>> c.simple_root(1)
[1, -1, 0, 0, 0]'

def simple_root(self, i):

    return self.basic_root((i - 1), i)
