Isotropic N-Point Basis Functions and Their Properties
  Isotropic functions of positions $\hat{\bf r}_1, \hat{\bf r}_2,\ldots,
\hat{\bf r}_N$, i.e. functions invariant under simultaneous rotations of all
the coordinates, are conveniently formed using spherical harmonics and
Clebsch-Gordan coefficients. An orthonormal basis of such functions provides a
formalism suitable for analyzing isotropic distributions such as those that
arise in cosmology, for instance in the clustering of galaxies as revealed by
large-scale structure surveys. The algebraic properties of the basis functions
are conveniently expressed in terms of 6-$j$ and 9-$j$ symbols. The calculation
of relations among the basis functions is facilitated by "Yutsis" diagrams for
the addition and recoupling of angular momenta.
