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Some properties of Wigner $3j$ coefficients: non-trivial zeros and connections to hypergeometric functions

The contribution of Jacques Raynal to angular-momentum theory is highly valuable. In the present article, I intend to recall the main aspects of his work related to Wigner $3j$ symbols. It is well known that the latter can be expressed with a hypergeometric series. The polynomial zeros of the $3j$ coefficients were initially characterized by the number of terms of the series minus one, which is the degree of the coefficient. A detailed study of the zeros of the $3j$ coefficient with respect to the degree $n$ for $J = a + b + c \leq 240$ ($a$, $b$ and $c$ being the angular momenta in the first line of the $3j$ symbol) by Raynal revealed that most zeros of high degree had small magnetic quantum numbers. This led him to define the order $m$ to improve the classification of the zeros of the $3j$ coefficient. Raynal did a search for the polynomial zeros of degree 1 to 7 and found that the number of zeros of degree 1 and 2 are infinite, though the number of zeros of degree larger than 3 decreases very quickly as the degree increases. Based on Whipple's transformations of hypergeometric $_3F_2$ functions with unit argument, Raynal generalized the Wigner $3j$ symbols to any arguments and pointed out that there are twelve sets of ten formulas (twelve sets of 120 generalized $3j$ symbols) which are equivalent in the usual case. In this paper, we also discuss other aspects of the zeros of $3j$ coefficients, such as the role of Diophantine equations and powerful numbers, or the alternative approach involving Labarthe patterns.