Path Algebra
If Γ is a quiver, then a path in Γ is a sequence of arrows an an-1 ... a3 a2 a1 such that the head of ai+1 = tail of ai, using the convention of concatenating paths from right to left.
If K is a field then the quiver algebra or path algebra KΓ is defined as a vector space having all the paths (of length ≥ 0) in the quiver as basis (including, for each vertex i of the quiver Γ, a trivial path of length 0; these paths are not assumed to be equal for different i), and multiplication given by concatenation of paths. If two paths cannot be concatenated because the end vertex of the first is not equal to the starting vertex of the second, their product is defined to be zero. This defines an associative algebra over K. This algebra has a unit element if and only if the quiver has only finitely many vertices. In this case, the modules over KΓ are naturally identified with the representations of Γ.
If the quiver has finitely many vertices and arrows, and the end vertex and starting vertex of any path are always distinct (i.e. Q has no oriented cycles), then KΓ is a finite-dimensional hereditary algebra over K.
Read more about this topic: Quiver (mathematics)
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