Triangle Coefficient

A function of three variables written $\Delta(abc)\equiv\Delta(a,b,c)$ and defined by

$$\Delta(abc)\equiv \sqrt{(a+b-c)!(a-b+c)!(-a+b+c)!\over (a+b+c+1)!}\,.$$

References

Shore, B. W. and Menzel, D. H. Principles of Atomic Spectra. New York: Wiley, p. 273, 1968.