from __future__ import print_function, division
from .cartan_type import Standard_Cartan
from sympy.core.compatibility import range
from sympy.matrices import eye
[docs]class TypeB(Standard_Cartan):
def __new__(cls, n):
if n < 2:
raise ValueError("n can not be less than 2")
return Standard_Cartan.__new__(cls, "B", n)
[docs] def dimension(self):
"""Dimension of the vector space V underlying the Lie algebra
Examples
========
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("B3")
>>> c.dimension()
3
"""
return self.n
[docs] def basic_root(self, i, j):
"""
This is a method just to generate roots
with a 1 iin the ith position and a -1
in the jth postion.
"""
root = [0]*self.n
root[i] = 1
root[j] = -1
return root
[docs] def simple_root(self, i):
"""
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 B_n the first n-1 simple roots are the same as the
roots in A_(n-1) (a 1 in the ith position, a -1 in
the (i+1)th position, and zeroes elsewhere). The n-th
simple root is the root with a 1 in the nth position
and zeroes elsewhere.
This method returns the ith simple root for the B series.
Examples
========
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("B3")
>>> c.simple_root(2)
[0, 1, -1]
"""
n = self.n
if i < n:
return self.basic_root(i-1, i)
else:
root = [0]*self.n
root[n-1] = 1
return root
[docs] def positive_roots(self):
"""
This method generates all the positive roots of
A_n. This is half of all of the roots of B_n;
by multiplying all the positive roots by -1 we
get the negative roots.
Examples
========
>>> from sympy.liealgebras.cartan_type import CartanType
>>> c = CartanType("A3")
>>> c.positive_roots()
{1: [1, -1, 0, 0], 2: [1, 0, -1, 0], 3: [1, 0, 0, -1], 4: [0, 1, -1, 0],
5: [0, 1, 0, -1], 6: [0, 0, 1, -1]}
"""
n = self.n
posroots = {}
k = 0
for i in range(0, n-1):
for j in range(i+1, n):
k += 1
posroots[k] = self.basic_root(i, j)
k += 1
root = self.basic_root(i, j)
root[j] = 1
posroots[k] = root
for i in range(0, n):
k += 1
root = [0]*n
root[i] = 1
posroots[k] = root
return posroots
[docs] def roots(self):
"""
Returns the total number of roots for B_n"
"""
n = self.n
return 2*(n**2)
[docs] def cartan_matrix(self):
"""
Returns the Cartan matrix for B_n.
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('B4')
>>> c.cartan_matrix()
Matrix([
[ 2, -1, 0, 0],
[-1, 2, -1, 0],
[ 0, -1, 2, -2],
[ 0, 0, -1, 2]])
"""
n = self.n
m = 2* eye(n)
i = 1
while i < n-1:
m[i, i+1] = -1
m[i, i-1] = -1
i += 1
m[0, 1] = -1
m[n-2, n-1] = -2
m[n-1, n-2] = -1
return m
[docs] def basis(self):
"""
Returns the number of independent generators of B_n
"""
n = self.n
return (n**2 - n)/2
[docs] def lie_algebra(self):
"""
Returns the Lie algebra associated with B_n
"""
n = self.n
return "so(" + str(2*n) + ")"
def dynkin_diagram(self):
n = self.n
diag = "---".join("0" for i in range(1, n)) + "=>=0\n"
diag += " ".join(str(i) for i in range(1, n+1))
return diag
