Exact expressions for the number of levels in single-\textit{j} orbits for three, four and five fermions

Abstract

We propose closed-form expressions of the distributions of magnetic quantum number $M$ and total angular momentum $J$ for three and four fermions in single-$j$ orbits. The latter formulas consist of polynomials with coefficients satisfying congruence properties. Such results, derived using doubly-recursive relations over $j$ and the number of fermions, enable us to deduce explicit expressions for the total number of levels in the case of three-, four- and five-fermion systems. We present applications of these formulas, such as sum rules for six-$j$ and nine-$j$ symbols, obtained from the connection with fractional-parentage coefficients, an alternative proof of the Ginocchio-Haxton relation or cancellation properties of the number of levels with a given angular momentum.