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In mathematics, the term irreducible is used in several ways.

  • A directed graph is irreducible if, given any two vertices, there exists a path from the first vertex to the second. A digraph is irreducible iff its adjacency matrix is irreducible.
  • In the theory of manifolds, an n-manifold is irreducible if any embedded (n−1)-sphere bounds an embedded n-ball. Implicit in this definition is the use of a suitable category, such as the category of differentiable manifolds or the category of piecewise-linear manifolds.

    The notions of irreducibility in algebra and manifold theory are related. An n-manifold is called prime, if it cannot be written as a connected sum of two n-manifolds (neither of which is an n-sphere). An irreducible manifold is thus prime, although the converse does not hold. From an algebraist's perspective, prime manifolds should be called "irreducible"; however, the topologist (in particular the 3-manifold topologist) finds the definition above more useful. The only compact, connected 3-manifolds that are prime but not irreducible are the trivial 2-sphere bundle over S1 and the twisted 2-sphere bundle over S1.

  • A matrix is irreducible if it cannot be made block upper triangular via a matrix permutation.
  • In universal algebra, irreducible can refer to the inability to represent an algebraic structure as a composition of simpler structures using a product construction; for example subdirectly irreducible.