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19 Elliptic IntegralsApplications

§19.32 Conformal Map onto a Rectangle

The function

19.32.1 z(p)=R_{F}\left(p-x_{1},p-x_{2},p-x_{3}\right),

with x_{1},x_{2},x_{3} real constants, has differential

19.32.2 \,\mathrm{d}z=-\frac{1}{2}\left(\prod_{j=1}^{3}(p-x_{j})^{-1/2}\right)\,%
\mathrm{d}p,\Im p>0; 0<\operatorname{ph}\left(p-x_{j}\right)<\pi, j=1,2,3.

If

19.32.3 x_{1}>x_{2}>x_{3},

then z(p) is a Schwartz–Christoffel mapping of the open upper-half p-plane onto the interior of the rectangle in the z-plane with vertices

19.32.4
z(\infty)=0,
z(x_{1})=R_{F}\left(0,x_{1}-x_{2},x_{1}-x_{3}\right)\quad\text{($>0$)},
z(x_{2})=z(x_{1})+z(x_{3}),
z(x_{3})=R_{F}\left(x_{3}-x_{1},x_{3}-x_{2},0\right)=-iR_{F}\left(0,x_{1}-x_{3%
},x_{2}-x_{3}\right).

As p proceeds along the entire real axis with the upper half-plane on the right, z describes the rectangle in the clockwise direction; hence z(x_{3}) is negative imaginary.

For further connections between elliptic integrals and conformal maps, see Bowman (1953, pp. 44–85).