# Group contraction

In theoretical physics, Eugene Wigner and Erdal İnönü have discussed[1] the possibility to obtain from a given Lie group a different (non-isomorphic) Lie group by a group contraction with respect to a continuous subgroup of it. That amounts to a limiting operation on a parameter of the Lie algebra, altering the structure constants of this Lie algebra in a nontrivial (singular) manner, under suitable circumstances.[2][3]

For example, the Lie algebra of SO(3), [X1,X2] = X3, etc, may be rewritten by a change of variables Y1 = εX1, Y2 = εX2, Y3 = X3, as

[Y1,Y2] = ε2 Y3,     [Y2,Y3] = Y1,     [Y3,Y1] = Y2.

The contraction limit ε → 0 trivializes the first commutator and thus yields the non-isomorphic algebra of the plane Euclidean group, E2 ~ ISO(2). (This is isomorphic to the cylindrical group, describing motions of a point on the surface of a cylinder. It is the little group of null 4-vectors.) Specifically, the translation generators Y1, Y2, now generate the Abelian normal subgroup of E2 (cf. Group extension), the parabolic Lorentz transformations.

Similar limits, of considerable application in physics (cf. Correspondence principles), contract

## References

1. ^ E. İnönü, E.P. Wigner (1953). "On the Contraction of Groups and Their Representations". Proc. Nat. Acad. Sci. 39 (6): 510–24. Bibcode:1953PNAS...39..510I. doi:10.1073/pnas.39.6.510. PMC 1063815. PMID 16589298.
2. ^ Segal, I. E. (1951). "A class of operator algebras which are determined by groups". Duke Mathematical Journal 18: 221. doi:10.1215/S0012-7094-51-01817-0.
3. ^ Saletan, E. J. (1961). "Contraction of Lie Groups". Journal of Mathematical Physics 2: 1–1. Bibcode:1961JMP.....2....1S. doi:10.1063/1.1724208.
4. ^ Gilmore, Robert (2006). Lie Groups, Lie Algebras, and Some of Their Applications (Dover Books on Mathematics), ISBN 0486445291 MR 1275599