# Dehn invariant

An invariant of polyhedra in three-dimensional space that decides whether two polyhedra of the same volume are "scissors congruent" (see Equal content and equal shape, figures of; Hilbert problems; Polyhedron).

Quite generally, a scissors-congruence invariant assigns to a polytope in space an element in a group such that , if is degenerate, and if there is a motion of the space such that .

For the Dehn invariant, the group chosen is the tensor product . To a polytope with edges one associates the element , where is the length of and is the dihedral angle of the planes meeting at . The Sydler theorem states that two polytopes in three-dimensional space are scissors equivalent if and only if they have equal volume and the same Dehn invariant, thus solving Hilbert's third problem in a very precise manner (cf. also Hilbert problems).

For higher dimensions there is a generalization, called the Hadwiger invariant or Dehn–Hadwiger invariant. This has given results in dimension four, [a4], and for the case when the group consists of translations only, [a2].

#### References

[a1] | P. Cartier, "Decomposition des polyèdres: le point sur le troisième problème de Hilbert" Sem. Bourbaki , 1984/5 (1986) pp. 261–288 |

[a2] | C.H. Sah, "Hilbert's third problem: scissors congruence" , Pitman (1979) |

[a3] | V.G. Boltianskii, "Hilbert's third problem" , Wiley (1978) |

[a4] | B. Jessen, "Zur Algebra der Polytope" Göttinger Nachrichte Math. Phys. (1972) pp. 47–53 |

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Dehn invariant.

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