# Path Algebras¶

class sage.quivers.algebra.PathAlgebra(k, P, order='negdegrevlex')

Create the path algebra of a quiver over a given field.

Given a quiver $$Q$$ and a field $$k$$, the path algebra $$kQ$$ is defined as follows. As a vector space it has basis the set of all paths in $$Q$$. Multiplication is defined on this basis and extended bilinearly. If $$p$$ is a path with terminal vertex $$t$$ and $$q$$ is a path with initial vertex $$i$$ then the product $$p*q$$ is defined to be the composition of the paths $$p$$ and $$q$$ if $$t = i$$ and $$0$$ otherwise.

INPUT:

• k – field (or commutative ring), the base field of the path algebra
• P – the path semigroup of a quiver $$Q$$
• order – optional string, one of “negdegrevlex” (default), “degrevlex”, “negdeglex” or “deglex”, defining the monomial order to be used.

OUTPUT:

• the path algebra $$kP$$ with the given monomial order

NOTE:

Monomial orders that are not degree orders are not supported.

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b']}}).path_semigroup()
sage: A = P.algebra(GF(7))
sage: A
Path algebra of Multi-digraph on 3 vertices over Finite Field of size 7
sage: A.variable_names()
('e_1', 'e_2', 'e_3', 'a', 'b')


Note that path algebras are uniquely defined by their quiver, field and monomial order:

sage: A is P.algebra(GF(7))
True
sage: A is P.algebra(GF(7), order="degrevlex")
False
sage: A is P.algebra(RR)
False
sage: A is DiGraph({1:{2:['a']}}).path_semigroup().algebra(GF(7))
False


The path algebra of an acyclic quiver has a finite basis:

sage: A.dimension()
6
sage: list(A.basis())
[e_1, e_2, e_3, a, b, a*b]


The path algebra can create elements from paths or from elements of the base ring:

sage: A(5)
5*e_1 + 5*e_2 + 5*e_3
sage: S = A.semigroup()
sage: S
Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices
sage: p = S([(1, 2, 'a')])
sage: r = S([(2, 3, 'b')])
sage: e2 = S([(2, 2)])
sage: x = A(p) + A(e2)
sage: x
a + e_2
sage: y = A(p) + A(r)
sage: y
a + b


Path algebras are graded algebras. The grading is given by assigning to each basis element the length of the path corresponding to that basis element:

sage: x.is_homogeneous()
False
sage: x.degree()
Traceback (most recent call last):
...
ValueError: Element is not homogeneous.
sage: y.is_homogeneous()
True
sage: y.degree()
1
sage: A[1]
Free module spanned by [a, b] over Finite Field of size 7
sage: A[2]
Free module spanned by [a*b] over Finite Field of size 7

Element
arrows()

Return the arrows of this algebra (corresponding to edges of the underlying quiver).

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b', 'c']}, 4:{}}).path_semigroup()
sage: A = P.algebra(GF(5))
sage: A.arrows()
(a, b, c)

degree_on_basis(x)

Return x.degree().

This function is here to make some methods work that are inherited from CombinatorialFreeModule.

EXAMPLES:

sage: A = DiGraph({0:{1:['a'], 2:['b']}, 1:{0:['c'], 1:['d']}, 2:{0:['e'],2:['f']}}).path_semigroup().algebra(ZZ)
sage: A.inject_variables()
Defining e_0, e_1, e_2, a, b, c, d, e, f
sage: X = a+2*b+3*c*e-a*d+5*e_0+3*e_2
sage: X
5*e_0 + a - a*d + 2*b + 3*e_2
sage: X.homogeneous_component(0)   # indirect doctest
5*e_0 + 3*e_2
sage: X.homogeneous_component(1)
a + 2*b
sage: X.homogeneous_component(2)
-a*d
sage: X.homogeneous_component(3)
0

gen(i)

Return the $$i$$-th generator of this algebra.

This is an idempotent (corresponding to a trivial path at a vertex) if $$i < n$$ (where $$n$$ is the number of vertices of the quiver), and a single-edge path otherwise.

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b', 'c']}, 4:{}}).path_semigroup()
sage: A = P.algebra(GF(5))
sage: A.gens()
(e_1, e_2, e_3, e_4, a, b, c)
sage: A.gen(2)
e_3
sage: A.gen(5)
b

gens()

Return the generators of this algebra (idempotents and arrows).

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b', 'c']}, 4:{}}).path_semigroup()
sage: A = P.algebra(GF(5))
sage: A.variable_names()
('e_1', 'e_2', 'e_3', 'e_4', 'a', 'b', 'c')
sage: A.gens()
(e_1, e_2, e_3, e_4, a, b, c)

homogeneous_component(n)

Return the $$n$$-th homogeneous piece of the path algebra.

INPUT:

• n – integer

OUTPUT:

EXAMPLES:

sage: P = DiGraph({1:{2:['a'], 3:['b']}, 2:{4:['c']}, 3:{4:['d']}}).path_semigroup()
sage: A = P.algebra(GF(7))
sage: A.homogeneous_component(2)
Free module spanned by [a*c, b*d] over Finite Field of size 7

sage: D = DiGraph({1: {2: 'a'}, 2: {3: 'b'}, 3: {1: 'c'}})
sage: P = D.path_semigroup()
sage: A = P.algebra(ZZ)
sage: A.homogeneous_component(3)
Free module spanned by [a*b*c, b*c*a, c*a*b] over Integer Ring

homogeneous_components()

Return the non-zero homogeneous components of self.

EXAMPLES:

sage: Q = DiGraph([[1,2,'a'],[2,3,'b'],[3,4,'c']])
sage: PQ = Q.path_semigroup()
sage: A = PQ.algebra(GF(7))
sage: A.homogeneous_components()
[Free module spanned by [e_1, e_2, e_3, e_4] over Finite Field of size 7,
Free module spanned by [a, b, c] over Finite Field of size 7,
Free module spanned by [a*b, b*c] over Finite Field of size 7,
Free module spanned by [a*b*c] over Finite Field of size 7]


Warning

Backward incompatible change: since trac ticket #12630 and until trac ticket #8678, this feature was implemented under the syntax list(A) by means of A.__iter__. This was incorrect since A.__iter__, when defined for a parent, should iterate through the elements of $$A$$.

idempotents()

Return the idempotents of this algebra (corresponding to vertices of the underlying quiver).

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b', 'c']}, 4:{}}).path_semigroup()
sage: A = P.algebra(GF(5))
sage: A.idempotents()
(e_1, e_2, e_3, e_4)

ngens()

Number of generators of this algebra.

EXAMPLES:

sage: P = DiGraph({1:{2:['a']}, 2:{3:['b', 'c']}, 4:{}}).path_semigroup()
sage: A = P.algebra(GF(5))
sage: A.ngens()
7

one()

Return the multiplicative identity element.

The multiplicative identity of a path algebra is the sum of the basis elements corresponding to the trivial paths at each vertex.

EXAMPLES:

sage: A = DiGraph({1:{2:['a']}, 2:{3:['b']}}).path_semigroup().algebra(QQ)
sage: A.one()
e_1 + e_2 + e_3

order_string()

Return the string that defines the monomial order of this algebra.

EXAMPLES:

sage: P1 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'))
sage: P2 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="degrevlex")
sage: P3 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="negdeglex")
sage: P4 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="deglex")
sage: P1.order_string()
'negdegrevlex'
sage: P2.order_string()
'degrevlex'
sage: P3.order_string()
'negdeglex'
sage: P4.order_string()
'deglex'

quiver()

Return the quiver from which the algebra self was formed.

OUTPUT:

EXAMPLES:

sage: P = DiGraph({1:{2:['a', 'b']}}).path_semigroup()
sage: A = P.algebra(GF(3))
sage: A.quiver() is P.quiver()
True

semigroup()

Return the (partial) semigroup from which the algebra self was constructed.

Note

The partial semigroup is formed by the paths of a quiver, multiplied by concatenation. If the quiver has more than a single vertex, then multiplication in the path semigroup is not always defined.

OUTPUT:

• the path semigroup from which self was formed (a partial semigroup)

EXAMPLES:

sage: P = DiGraph({1:{2:['a', 'b']}}).path_semigroup()
sage: A = P.algebra(GF(3))
sage: A.semigroup() is P
True

sum(iter_of_elements)

Returns the sum of all elements in iter_of_elements

INPUT:

• iter_of_elements: iterator of elements of self

NOTE:

It overrides a method inherited from CombinatorialFreeModule, which relies on a private attribute of elements—an implementation detail that is simply not available for PathAlgebraElement.

EXAMPLES:

sage: A = DiGraph({0:{1:['a'], 2:['b']}, 1:{0:['c'], 1:['d']}, 2:{0:['e'],2:['f']}}).path_semigroup().algebra(ZZ)
sage: A.inject_variables()
Defining e_0, e_1, e_2, a, b, c, d, e, f
sage: A.sum((a, 2*b, 3*c*e, -a*d, 5*e_0, 3*e_2))
5*e_0 + a - a*d + 2*b + 3*e_2