In mathematics, a Casimir invariant of a Lie algebra is a member of the center of the universal enveloping algebra of the Lie algebra. In other words, it is a member of the algebra that commutes with all the generators in the Lie algebra.
The number of independent elements of the center of the universal enveloping algebra is also the rank in the case of a semisimple Lie algebra.
In any irreducible representation of the Lie algebra, by Schur's Lemma, any member of the center of the universal enveloping algebra commutes with everything and thus is proportional to the identity. This constant of proportionality can be used to classify the representations of the Lie algebra (and hence, also of its Lie group). Mass and spin are examples of these constants, as are many other quantum numbers found in quantum mechanics. Topological quantum numbers are a counterexample.
Casimir Edward Norrisey of Charlottesville died Monday, January 14, 2002, at Martha Jefferson Hospital, 17 days shy of his 79th birthday.
Casimir, known as Cas or Casey, served the Catholic community of Charlottesville in many capacities.
He was an active amateur radio operator with the call letters K4MTX, and a member of the Quarter Century Wireless Association, the American Radio Relay League, and the Albemarle Amateur Radio Club.
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