Carlson symmetric form

When any two, or all three of the arguments of ${\displaystyle R_{F}}$ are the same, then a substitution of ${\displaystyle {\sqrt {t+x}}=u}$ renders the integrand rational. The integral can then be expressed in terms of elementary transcendental functions.

${\displaystyle R_{C}(x,y)=R_{F}(x,y,y)={\frac {1}{2}}\int _{0}^{\infty }{\frac {1}{{\sqrt {t+x}}(t+y)}}dt=\int _{\sqrt {x}}^{\infty }{\frac {1}{u^{2}-x+y}}du={\begin{cases}{\frac {\arccos {\sqrt {\frac {x}{y}}}}{\sqrt {y-x}}},&x<y\\{\frac {1}{\sqrt {y}}},&x=y\\{\frac {\mathrm {arccosh} {\sqrt {\frac {x}{y}}}}{\sqrt {x-y}}},&x>y\\\end{cases}}}$

Similarly, when at least two of the first three arguments of ${\displaystyle R_{J}}$ are the same,

${\displaystyle R_{J}(x,y,y,p)=3\int _{\sqrt {x}}^{\infty }{\frac {1}{(u^{2}-x+y)(u^{2}-x+p)}}du={\begin{cases}{\frac {3}{p-y}}(R_{C}(x,y)-R_{C}(x,p)),&y\neq p\\{\frac {3}{2(y-x)}}\left(R_{C}(x,y)-{\frac {1}{y}}{\sqrt {x}}\right),&y=p\neq x\\{\frac {1}{y^{\frac {3}{2}}}},&y=p=x\\\end{cases}}}$

In obtaining a Taylor series expansion for ${\displaystyle \scriptstyle {R_{F}}}$ or ${\displaystyle \scriptstyle {R_{J}}}$ it proves convenient to expand about the mean value of the several arguments. So for ${\displaystyle \scriptstyle {R_{F}}}$, letting the mean value of the arguments be ${\displaystyle \scriptstyle {A=(x+y+z)/3}}$, and using homogeneity, define ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$ and ${\displaystyle \scriptstyle {\Delta z}}$ by

${\displaystyle {\begin{aligned}R_{F}(x,y,z)&=R_{F}(A(1-\Delta x),A(1-\Delta y),A(1-\Delta z))\\&={\frac {1}{\sqrt {A}}}R_{F}(1-\Delta x,1-\Delta y,1-\Delta z)\end{aligned}}}$

that is ${\displaystyle \scriptstyle {\Delta x=1-x/A}}$ etc. The differences ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$ and ${\displaystyle \scriptstyle {\Delta z}}$ are defined with this sign (such that they are subtracted), in order to be in agreement with Carlson's papers. Since ${\displaystyle \scriptstyle {R_{F}(x,y,z)}}$ is symmetric under permutation of ${\displaystyle \scriptstyle {x}}$, ${\displaystyle \scriptstyle {y}}$ and ${\displaystyle \scriptstyle {z}}$, it is also symmetric in the quantities ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$ and ${\displaystyle \scriptstyle {\Delta z}}$. It follows that both the integrand of ${\displaystyle \scriptstyle {R_{F}}}$ and its integral can be expressed as functions of the elementary symmetric polynomials in ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$ and ${\displaystyle \scriptstyle {\Delta z}}$ which are

${\displaystyle E_{1}=\Delta x+\Delta y+\Delta z=0}$

${\displaystyle E_{2}=\Delta x\Delta y+\Delta y\Delta z+\Delta z\Delta x}$

${\displaystyle E_{3}=\Delta x\Delta y\Delta z}$

Expressing the integrand in terms of these polynomials, performing a multidimensional Taylor expansion and integrating term-by-term...

${\displaystyle {\begin{aligned}R_{F}(x,y,z)&={\frac {1}{2{\sqrt {A}}}}\int _{0}^{\infty }{\frac {1}{\sqrt {(t+1)^{3}-(t+1)^{2}E_{1}+(t+1)E_{2}-E_{3}}}}dt\\&={\frac {1}{2{\sqrt {A}}}}\int _{0}^{\infty }\left({\frac {1}{(t+1)^{\frac {3}{2}}}}-{\frac {E_{2}}{2(t+1)^{\frac {7}{2}}}}+{\frac {E_{3}}{2(t+1)^{\frac {9}{2}}}}+{\frac {3E_{2}^{2}}{8(t+1)^{\frac {11}{2}}}}-{\frac {3E_{2}E_{3}}{4(t+1)^{\frac {13}{2}}}}+O(E_{1})+O(\Delta ^{6})\right)dt\\&={\frac {1}{\sqrt {A}}}\left(1-{\frac {1}{10}}E_{2}+{\frac {1}{14}}E_{3}+{\frac {1}{24}}E_{2}^{2}-{\frac {3}{44}}E_{2}E_{3}+O(E_{1})+O(\Delta ^{6})\right)\end{aligned}}}$

The advantage of expanding about the mean value of the arguments is now apparent; it reduces ${\displaystyle \scriptstyle {E_{1}}}$ identically to zero, and so eliminates all terms involving ${\displaystyle \scriptstyle {E_{1}}}$ - which otherwise would be the most numerous.

An ascending series for ${\displaystyle \scriptstyle {R_{J}}}$ may be found in a similar way. There is a slight difficulty because ${\displaystyle \scriptstyle {R_{J}}}$ is not fully symmetric; its dependence on its fourth argument, ${\displaystyle \scriptstyle {p}}$, is different from its dependence on ${\displaystyle \scriptstyle {x}}$, ${\displaystyle \scriptstyle {y}}$ and ${\displaystyle \scriptstyle {z}}$. This is overcome by treating ${\displaystyle \scriptstyle {R_{J}}}$ as a fully symmetric function of five arguments, two of which happen to have the same value ${\displaystyle \scriptstyle {p}}$. The mean value of the arguments is therefore taken to be

${\displaystyle A={\frac {x+y+z+2p}{5}}}$

and the differences ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$ ${\displaystyle \scriptstyle {\Delta z}}$ and ${\displaystyle \scriptstyle {\Delta p}}$ defined by

${\displaystyle {\begin{aligned}R_{J}(x,y,z,p)&=R_{J}(A(1-\Delta x),A(1-\Delta y),A(1-\Delta z),A(1-\Delta p))\\&={\frac {1}{A^{\frac {3}{2}}}}R_{J}(1-\Delta x,1-\Delta y,1-\Delta z,1-\Delta p)\end{aligned}}}$

The elementary symmetric polynomials in ${\displaystyle \scriptstyle {\Delta x}}$, ${\displaystyle \scriptstyle {\Delta y}}$, ${\displaystyle \scriptstyle {\Delta z}}$, ${\displaystyle \scriptstyle {\Delta p}}$ and (again) ${\displaystyle \scriptstyle {\Delta p}}$ are in full

${\displaystyle E_{1}=\Delta x+\Delta y+\Delta z+2\Delta p=0}$

${\displaystyle E_{2}=\Delta x\Delta y+\Delta y\Delta z+2\Delta z\Delta p+\Delta p^{2}+2\Delta p\Delta x+\Delta x\Delta z+2\Delta y\Delta p}$

${\displaystyle E_{3}=\Delta z\Delta p^{2}+\Delta x\Delta p^{2}+2\Delta x\Delta y\Delta p+\Delta x\Delta y\Delta z+2\Delta y\Delta z\Delta p+\Delta y\Delta p^{2}+2\Delta x\Delta z\Delta p}$

${\displaystyle E_{4}=\Delta y\Delta z\Delta p^{2}+\Delta x\Delta z\Delta p^{2}+\Delta x\Delta y\Delta p^{2}+2\Delta x\Delta y\Delta z\Delta p}$

${\displaystyle E_{5}=\Delta x\Delta y\Delta z\Delta p^{2}}$

However, it is possible to simplify the formulae for ${\displaystyle \scriptstyle {E_{2}}}$, ${\displaystyle \scriptstyle {E_{3}}}$ and ${\displaystyle \scriptstyle {E_{4}}}$ using the fact that ${\displaystyle \scriptstyle {E_{1}=0}}$. Expressing the integrand in terms of these polynomials, performing a multidimensional Taylor expansion and integrating term-by-term as before...

${\displaystyle {\begin{aligned}R_{J}(x,y,z,p)&={\frac {3}{2A^{\frac {3}{2}}}}\int _{0}^{\infty }{\frac {1}{\sqrt {(t+1)^{5}-(t+1)^{4}E_{1}+(t+1)^{3}E_{2}-(t+1)^{2}E_{3}+(t+1)E_{4}-E_{5}}}}dt\\&={\frac {3}{2A^{\frac {3}{2}}}}\int _{0}^{\infty }\left({\frac {1}{(t+1)^{\frac {5}{2}}}}-{\frac {E_{2}}{2(t+1)^{\frac {9}{2}}}}+{\frac {E_{3}}{2(t+1)^{\frac {11}{2}}}}+{\frac {3E_{2}^{2}-4E_{4}}{8(t+1)^{\frac {13}{2}}}}+{\frac {2E_{5}-3E_{2}E_{3}}{4(t+1)^{\frac {15}{2}}}}+O(E_{1})+O(\Delta ^{6})\right)dt\\&={\frac {1}{A^{\frac {3}{2}}}}\left(1-{\frac {3}{14}}E_{2}+{\frac {1}{6}}E_{3}+{\frac {9}{88}}E_{2}^{2}-{\frac {3}{22}}E_{4}-{\frac {9}{52}}E_{2}E_{3}+{\frac {3}{26}}E_{5}+O(E_{1})+O(\Delta ^{6})\right)\end{aligned}}}$

As with ${\displaystyle \scriptstyle {R_{J}}}$, by expanding about the mean value of the arguments, more than half the terms (those involving ${\displaystyle \scriptstyle {E_{1}}}$) are eliminated.

In general, the arguments x, y, z of Carlson's integrals may not be real and negative, as this would place a branch point on the path of integration, making the integral ambiguous. However, if the second argument of ${\displaystyle \scriptstyle {R_{C}}}$, or the fourth argument, p, of ${\displaystyle \scriptstyle {R_{J}}}$ is negative, then this results in a simple pole on the path of integration. In these cases the Cauchy principal value (finite part) of the integrals may be of interest; these are

${\displaystyle \mathrm {p.v.} \;R_{C}(x,-y)={\sqrt {\frac {x}{x+y}}}\,R_{C}(x+y,y),}$

and

${\displaystyle {\begin{aligned}\mathrm {p.v.} \;R_{J}(x,y,z,-p)&={\frac {(q-y)R_{J}(x,y,z,q)-3R_{F}(x,y,z)+3{\sqrt {y}}R_{C}(xz,-pq)}{y+p}}\\&={\frac {(q-y)R_{J}(x,y,z,q)-3R_{F}(x,y,z)+3{\sqrt {\frac {xyz}{xz+pq}}}R_{C}(xz+pq,pq)}{y+p}}\end{aligned}}}$

where

${\displaystyle q=y+{\frac {(z-y)(y-x)}{y+p}}.}$

which must be greater than zero for ${\displaystyle \scriptstyle {R_{J}(x,y,z,q)}}$ to be evaluated. This may be arranged by permuting x, y and z so that the value of y is between that of x and z.

The duplication theorem can be used for a fast and robust evaluation of the Carlson symmetric form of elliptic integrals and therefore also for the evaluation of Legendre-form of elliptic integrals. Let us calculate ${\displaystyle R_{F}(x,y,z)}$: first, define ${\displaystyle x_{0}=x}$, ${\displaystyle y_{0}=y}$ and ${\displaystyle z_{0}=z}$. Then iterate the series

${\displaystyle \lambda _{n}={\sqrt {x_{n}}}{\sqrt {y_{n}}}+{\sqrt {y_{n}}}{\sqrt {z_{n}}}+{\sqrt {z_{n}}}{\sqrt {x_{n}}},}$

${\displaystyle x_{n+1}={\frac {x_{n}+\lambda _{n}}{4}},y_{n+1}={\frac {y_{n}+\lambda _{n}}{4}},z_{n+1}={\frac {z_{n}+\lambda _{n}}{4}}}$

until the desired precision is reached: if ${\displaystyle x}$, ${\displaystyle y}$ and ${\displaystyle z}$ are non-negative, all of the series will converge quickly to a given value, say, ${\displaystyle \mu }$. Therefore,

${\displaystyle R_{F}\left(x,y,z\right)=R_{F}\left(\mu ,\mu ,\mu \right)=\mu ^{-1/2}.}$

Evaluating ${\displaystyle R_{C}(x,y)}$ is much the same due to the relation

${\displaystyle R_{C}\left(x,y\right)=R_{F}\left(x,y,y\right).}$

• B. C. Carlson, John L. Gustafson 'Asymptotic approximations for symmetric elliptic integrals' 1993 arXiv
• B. C. Carlson 'Numerical Computation of Real Or Complex Elliptic Integrals' 1994 arXiv
• B. C. Carlson 'Elliptic Integrals:Symmetric Integrals' in Chap. 19 of Digital Library of Mathematical Functions. Release date 2010-05-07. National Institute of Standards and Technology.
• 'Profile: Bille C. Carlson' in Digital Library of Mathematical Functions. National Institute of Standards and Technology.
• Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (2007), "Section 6.12. Elliptic Integrals and Jacobian Elliptic Functions", Numerical Recipes: The Art of Scientific Computing (3rd ed.), New York: Cambridge University Press, ISBN 978-0-521-88068-8
• Fortran code from SLATEC for evaluating RF, RJ, RC, RD,