Several familiar special functions possess a hidden permutation symmetry which accounts for some of their transformation properties. For example the transformation of the elliptic integral $K(k)$ into $K({{ik} / {k'}})$ expresses the symmetry of an integral representing Gauss’ arithmetic-geometric mean of two variables. Likewise, a transformation of the hypergeometric function ${}_2 F_1 $ with argument z into the same function with argument ${z / {(z - 1)}}$ can be viewed as a symmetry under interchange of two variables, but in this case two parameters must be interchanged as well. A similar remark applies to Kummer’s transformation of the confluent hypergeometric function. The five transformations which change the modulus k of an incomplete elliptic integral into $k'$, ${1 / k}$, ${1 / {k'}}$, ${{ik} / {k'}}$, and ${{k'} / {ik}}$ are equivalent to the five nontrivial permutations of three variables, and the theory of elliptic integrals can be simplified by choosing standard integrals which are explicitly symmetric. Appell’s double hypergeometric function $F_1 $ has a partly concealed symmetry under simultaneous permutations of three parameters and three variables. The feature common to all these examples is a function $F(b_1 ,b_2 , \cdots ,b_k ;z_1 ,z_2 , \cdots ,z_k )$ which is symmetric in the indices $1,2, \cdots ,k$ of the parameters b and variables z and represents a weighted average of a function of one variable over the convex hull of $\{ {z_1 ,z_2 , \cdots ,z_k } \}$. The advantage of using explicitly symmetric functions is illustrated by generalizing Borchardt’s iterative algorithm for computing an inverse cosine to an algorithm for computing an inverse elliptic cosine.
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