Noncommutative pfaffians and classification of states of five-dimensional quasi-spin
Noncommutative pfaffians associated with an orthogonal algebra $\mathfrak{o}_N$ are some special elements of the universal enveloping algebra $U(\mathfrak{o}_N)$. Using pfaffians we construct the fourth quantum number which together with the naturally defined three quantum numbers allow to classify the states of a five-dimensional quasi-spin. The pfaffians are treated as creation operators for the new quantum number.
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