# §19.2 Definitions

## §19.2(i) General Elliptic Integrals

Let $s^{2}(t)$ be a cubic or quartic polynomial in $t$ with simple zeros, and let $r(s,t)$ be a rational function of $s$ and $t$ containing at least one odd power of $s$. Then

 19.2.1 $\int r(s,t)\mathrm{d}t$ Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral and $r(s,t)$: rational function Referenced by: §19.2(i) Permalink: http://dlmf.nist.gov/19.2.E1 Encodings: TeX, pMML, png See also: Annotations for 19.2(i)

is called an elliptic integral. Because $s^{2}$ is a polynomial, we have

 19.2.2 $r(s,t)=\frac{(p_{1}+p_{2}s)(p_{3}-p_{4}s)s}{(p_{3}+p_{4}s)(p_{3}-p_{4}s)s}=% \frac{\rho}{s}+\sigma,$ Symbols: $r(s,t)$: rational function, $\rho(t)$: rational function and $\sigma(t)$: rational function Permalink: http://dlmf.nist.gov/19.2.E2 Encodings: TeX, pMML, png See also: Annotations for 19.2(i)

where $p_{j}$ is a polynomial in $t$ while $\rho$ and $\sigma$ are rational functions of $t$. Thus the elliptic part of (19.2.1) is

 19.2.3 $\int\frac{\rho(t)}{s(t)}\mathrm{d}t.$ Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral and $\rho(t)$: rational function Referenced by: §19.14(ii), §19.16(ii), §19.29(ii) Permalink: http://dlmf.nist.gov/19.2.E3 Encodings: TeX, pMML, png See also: Annotations for 19.2(i)

## §19.2(ii) Legendre’s Integrals

Assume $1-{\mathop{\sin\/}\nolimits^{2}}\phi\in\mathbb{C}\setminus(-\infty,0]$ and $1-k^{2}{\mathop{\sin\/}\nolimits^{2}}\phi\in\mathbb{C}\setminus(-\infty,0]$, except that one of them may be 0, and $1-\alpha^{2}{\mathop{\sin\/}\nolimits^{2}}\phi\in\mathbb{C}\setminus\{0\}$. Then

 19.2.4 $\displaystyle\mathop{F\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\int_{0}^{\phi}\frac{\mathrm{d}\theta}{\sqrt{1-k^{2}{\mathop{% \sin\/}\nolimits^{2}}\theta}}=\int_{0}^{\mathop{\sin\/}\nolimits\phi}\frac{% \mathrm{d}t}{\sqrt{1-t^{2}}\sqrt{1-k^{2}t^{2}}},$ Defines: $\mathop{F\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the first kind Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function, $\phi$: real or complex argument and $k$: real or complex modulus Referenced by: §19.5, §19.6(ii) Permalink: http://dlmf.nist.gov/19.2.E4 Encodings: TeX, pMML, png See also: Annotations for 19.2(ii) 19.2.5 $\displaystyle\mathop{E\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\int_{0}^{\phi}\sqrt{1-k^{2}{\mathop{\sin\/}\nolimits^{2}}\theta% }\mathrm{d}\theta\\ =\int_{0}^{\mathop{\sin\/}\nolimits\phi}\frac{\sqrt{1-k^{2}t^{2}}}{\sqrt{1-t^{% 2}}}\mathrm{d}t.$ Defines: $\mathop{E\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the second kind Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function, $\phi$: real or complex argument and $k$: real or complex modulus Referenced by: §19.6(iii), §22.16(ii) Permalink: http://dlmf.nist.gov/19.2.E5 Encodings: TeX, pMML, png See also: Annotations for 19.2(ii) 19.2.6 $\displaystyle\mathop{D\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\int_{0}^{\phi}\frac{{\mathop{\sin\/}\nolimits^{2}}\theta\mathrm% {d}\theta}{\sqrt{1-k^{2}{\mathop{\sin\/}\nolimits^{2}}\theta}}=\int_{0}^{% \mathop{\sin\/}\nolimits\phi}\frac{t^{2}\mathrm{d}t}{\sqrt{1-t^{2}}\sqrt{1-k^{% 2}t^{2}}}=(\mathop{F\/}\nolimits\!\left(\phi,k\right)-\mathop{E\/}\nolimits\!% \left(\phi,k\right))/k^{2}.$ Defines: $\mathop{D\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: incomplete elliptic integral of Legendre’s type Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\mathop{F\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the first kind, $\mathop{E\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the second kind, $\int$: integral, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function, $\phi$: real or complex argument and $k$: real or complex modulus Referenced by: §19.25(i), §19.36(i) Permalink: http://dlmf.nist.gov/19.2.E6 Encodings: TeX, pMML, png See also: Annotations for 19.2(ii)
 19.2.7 $\mathop{\Pi\/}\nolimits\!\left(\phi,\alpha^{2},k\right)=\int_{0}^{\phi}\frac{% \mathrm{d}\theta}{\sqrt{1-k^{2}{\mathop{\sin\/}\nolimits^{2}}\theta}(1-\alpha^% {2}{\mathop{\sin\/}\nolimits^{2}}\theta)}=\int_{0}^{\mathop{\sin\/}\nolimits% \phi}\frac{\mathrm{d}t}{\sqrt{1-t^{2}}\sqrt{1-k^{2}t^{2}}(1-\alpha^{2}t^{2})}.$ Defines: $\mathop{\Pi\/}\nolimits\!\left(\NVar{\phi},\NVar{\alpha}^{2},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the third kind Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function, $\phi$: real or complex argument, $k$: real or complex modulus and $\alpha^{2}$: real or complex parameter Referenced by: §19.5 Permalink: http://dlmf.nist.gov/19.2.E7 Encodings: TeX, pMML, png See also: Annotations for 19.2(ii)

The paths of integration are the line segments connecting the limits of integration. The integral for $\mathop{E\/}\nolimits\!\left(\phi,k\right)$ is well defined if $k^{2}={\mathop{\sin\/}\nolimits^{2}}\phi=1$, and the Cauchy principal value (§1.4(v)) of $\mathop{\Pi\/}\nolimits\!\left(\phi,\alpha^{2},k\right)$ is taken if $1-\alpha^{2}{\mathop{\sin\/}\nolimits^{2}}\phi$ vanishes at an interior point of the integration path. Also, if $k^{2}$ and $\alpha^{2}$ are real, then $\mathop{\Pi\/}\nolimits\!\left(\phi,\alpha^{2},k\right)$ is called a circular or hyperbolic case according as $\alpha^{2}(\alpha^{2}-k^{2})(\alpha^{2}-1)$ is negative or positive. The circular and hyperbolic cases alternate in the four intervals of the real line separated by the points $\alpha^{2}=0,k^{2},1$.

The cases with $\phi=\pi/2$ are the complete integrals:

 19.2.8 $\displaystyle\mathop{K\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{F\/}\nolimits\!\left(\pi/2,k\right),$ $\displaystyle\mathop{E\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{E\/}\nolimits\!\left(\pi/2,k\right),$ $\displaystyle\mathop{D\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{D\/}\nolimits\!\left(\pi/2,k\right)=(\mathop{K\/}% \nolimits\!\left(k\right)-\mathop{E\/}\nolimits\!\left(k\right))/k^{2},$ $\displaystyle\mathop{\Pi\/}\nolimits\!\left(\alpha^{2},k\right)$ $\displaystyle=\mathop{\Pi\/}\nolimits\!\left(\pi/2,\alpha^{2},k\right),$ Defines: $\mathop{D\/}\nolimits\!\left(\NVar{k}\right)$: complete elliptic integral of Legendre’s type, $\mathop{K\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complete elliptic integral of the first kind, $\mathop{E\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complete elliptic integral of the second kind and $\mathop{\Pi\/}\nolimits\!\left(\NVar{\alpha}^{2},\NVar{k}\right)$: Legendre’s complete elliptic integral of the third kind Symbols: $\pi$: the ratio of the circumference of a circle to its diameter, $\mathop{F\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the first kind, $\mathop{E\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the second kind, $\mathop{D\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: incomplete elliptic integral of Legendre’s type, $\mathop{\Pi\/}\nolimits\!\left(\NVar{\phi},\NVar{\alpha}^{2},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the third kind, $k$: real or complex modulus and $\alpha^{2}$: real or complex parameter Permalink: http://dlmf.nist.gov/19.2.E8 Encodings: TeX, TeX, TeX, TeX, pMML, pMML, pMML, pMML, png, png, png, png See also: Annotations for 19.2(ii)
 19.2.9 $\displaystyle\mathop{{K^{\prime}}\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{K\/}\nolimits\!\left(k^{\prime}\right),$ $\displaystyle\mathop{{E^{\prime}}\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{E\/}\nolimits\!\left(k^{\prime}\right),$ $\displaystyle k^{\prime}$ $\displaystyle=\sqrt{1-k^{2}}.$ Defines: $\mathop{{K^{\prime}}\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complementary complete elliptic integral of the first kind and $\mathop{{E^{\prime}}\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complementary complete elliptic integral of the second kind Symbols: $\mathop{K\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complete elliptic integral of the first kind, $\mathop{E\/}\nolimits\!\left(\NVar{k}\right)$: Legendre’s complete elliptic integral of the second kind, $k$: real or complex modulus and $k^{\prime}$: complementary modulus Referenced by: §22.11 Permalink: http://dlmf.nist.gov/19.2.E9 Encodings: TeX, TeX, TeX, pMML, pMML, pMML, png, png, png See also: Annotations for 19.2(ii)

If $m$ is an integer, then

 19.2.10 $\displaystyle\mathop{F\/}\nolimits\!\left(m\pi\pm\phi,k\right)$ $\displaystyle=2m\mathop{K\/}\nolimits\!\left(k\right)\pm\mathop{F\/}\nolimits% \!\left(\phi,k\right),$ $\displaystyle\mathop{E\/}\nolimits\!\left(m\pi\pm\phi,k\right)$ $\displaystyle=2m\mathop{E\/}\nolimits\!\left(k\right)\pm\mathop{E\/}\nolimits% \!\left(\phi,k\right),$ $\displaystyle\mathop{D\/}\nolimits\!\left(m\pi\pm\phi,k\right)$ $\displaystyle=2m\mathop{D\/}\nolimits\!\left(k\right)\pm\mathop{D\/}\nolimits% \!\left(\phi,k\right).$

## §19.2(iii) Bulirsch’s Integrals

Bulirsch’s integrals are linear combinations of Legendre’s integrals that are chosen to facilitate computational application of Bartky’s transformation (Bartky (1938)). Two are defined by

 19.2.11 $\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},p,a,b\right)=\int_{0}^{\pi/2}% \frac{a{\mathop{\cos\/}\nolimits^{2}}\theta+b{\mathop{\sin\/}\nolimits^{2}}% \theta}{{\mathop{\cos\/}\nolimits^{2}}\theta+p{\mathop{\sin\/}\nolimits^{2}}% \theta}\frac{\mathrm{d}\theta}{\sqrt{{\mathop{\cos\/}\nolimits^{2}}\theta+k_{c% }^{2}{\mathop{\sin\/}\nolimits^{2}}\theta}},$ Defines: $\mathop{\mathrm{cel}\/}\nolimits\!\left(\NVar{k_{c}},\NVar{p},\NVar{a},\NVar{b% }\right)$: Bulirsch’s complete elliptic integral Symbols: $\pi$: the ratio of the circumference of a circle to its diameter, $\mathop{\cos\/}\nolimits\NVar{z}$: cosine function, $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function and $k$: real or complex modulus Referenced by: §19.2(iii) Permalink: http://dlmf.nist.gov/19.2.E11 Encodings: TeX, pMML, png See also: Annotations for 19.2(iii)
 19.2.12 $\mathop{\mathrm{el2}\/}\nolimits\!\left(x,k_{c},a,b\right)=\int_{0}^{\mathop{% \mathrm{arctan}\/}\nolimits x}\frac{a+b{\mathop{\tan\/}\nolimits^{2}}\theta}{% \sqrt{(1+{\mathop{\tan\/}\nolimits^{2}}\theta)(1+k_{c}^{2}{\mathop{\tan\/}% \nolimits^{2}}\theta)}}\mathrm{d}\theta.$ Defines: $\mathop{\mathrm{el2}\/}\nolimits\!\left(\NVar{x},\NVar{k_{c}},\NVar{a},\NVar{b% }\right)$: Bulirsch’s incomplete elliptic integral of the second kind Symbols: $\mathrm{d}\NVar{x}$: differential of $x$, $\int$: integral, $\mathop{\mathrm{arctan}\/}\nolimits\NVar{z}$: arctangent function, $\mathop{\tan\/}\nolimits\NVar{z}$: tangent function and $k$: real or complex modulus Permalink: http://dlmf.nist.gov/19.2.E12 Encodings: TeX, pMML, png See also: Annotations for 19.2(iii)

Here $a,b,p$ are real parameters, and $k_{c}$ and $x$ are real or complex variables, with $p\neq 0$, $k_{c}\neq 0$. If $-\infty, then the integral in (19.2.11) is a Cauchy principal value.

With

 19.2.13 $\displaystyle k_{c}$ $\displaystyle=k^{\prime},$ $\displaystyle p$ $\displaystyle=1-\alpha^{2},$ $\displaystyle x$ $\displaystyle=\mathop{\tan\/}\nolimits\phi,$

special cases include

 19.2.14 $\displaystyle\mathop{K\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},1,1,1\right),$ $\displaystyle\mathop{E\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},1,1,k_{c}^{2}% \right),$ $\displaystyle\mathop{D\/}\nolimits\!\left(k\right)$ $\displaystyle=\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},1,0,1\right),$ $\displaystyle(\mathop{E\/}\nolimits\!\left(k\right)-{k^{\prime}}^{2}\mathop{K% \/}\nolimits\!\left(k\right))/k^{2}$ $\displaystyle=\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},1,1,0\right),$ $\displaystyle\mathop{\Pi\/}\nolimits\!\left(\alpha^{2},k\right)$ $\displaystyle=\mathop{\mathrm{cel}\/}\nolimits\!\left(k_{c},p,1,1\right),$

and

 19.2.15 $\displaystyle\mathop{F\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\mathop{\mathrm{el1}\/}\nolimits\!\left(x,k_{c}\right)=\mathop{% \mathrm{el2}\/}\nolimits\!\left(x,k_{c},1,1\right),$ $\displaystyle\mathop{E\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\mathop{\mathrm{el2}\/}\nolimits\!\left(x,k_{c},1,k_{c}^{2}% \right),$ $\displaystyle\mathop{D\/}\nolimits\!\left(\phi,k\right)$ $\displaystyle=\mathop{\mathrm{el2}\/}\nolimits\!\left(x,k_{c},0,1\right).$ Defines: $\mathop{\mathrm{el1}\/}\nolimits\!\left(\NVar{x},\NVar{k_{c}}\right)$: Bulirsch’s incomplete elliptic integral of the first kind Symbols: $\mathop{\mathrm{el2}\/}\nolimits\!\left(\NVar{x},\NVar{k_{c}},\NVar{a},\NVar{b% }\right)$: Bulirsch’s incomplete elliptic integral of the second kind, $\mathop{F\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the first kind, $\mathop{E\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the second kind, $\mathop{D\/}\nolimits\!\left(\NVar{\phi},\NVar{k}\right)$: incomplete elliptic integral of Legendre’s type, $\phi$: real or complex argument and $k$: real or complex modulus Permalink: http://dlmf.nist.gov/19.2.E15 Encodings: TeX, TeX, TeX, pMML, pMML, pMML, png, png, png See also: Annotations for 19.2(iii)

The integrals are complete if $x=\infty$. If $1, then $k_{c}$ is pure imaginary.

Lastly, corresponding to Legendre’s incomplete integral of the third kind we have

 19.2.16 $\mathop{\mathrm{el3}\/}\nolimits\!\left(x,k_{c},p\right)=\int_{0}^{\mathop{% \mathrm{arctan}\/}\nolimits x}\frac{\mathrm{d}\theta}{({\mathop{\cos\/}% \nolimits^{2}}\theta+p{\mathop{\sin\/}\nolimits^{2}}\theta)\sqrt{{\mathop{\cos% \/}\nolimits^{2}}\theta+k_{c}^{2}{\mathop{\sin\/}\nolimits^{2}}\theta}}=% \mathop{\Pi\/}\nolimits\!\left(\mathop{\mathrm{arctan}\/}\nolimits x,1-p,k% \right),$ $x^{2}\neq-1/p$. Defines: $\mathop{\mathrm{el3}\/}\nolimits\!\left(\NVar{x},\NVar{k_{c}},\NVar{p}\right)$: Bulirsch’s incomplete elliptic integral of the third kind Symbols: $\mathop{\cos\/}\nolimits\NVar{z}$: cosine function, $\mathrm{d}\NVar{x}$: differential of $x$, $\mathop{\Pi\/}\nolimits\!\left(\NVar{\phi},\NVar{\alpha}^{2},\NVar{k}\right)$: Legendre’s incomplete elliptic integral of the third kind, $\int$: integral, $\mathop{\mathrm{arctan}\/}\nolimits\NVar{z}$: arctangent function, $\mathop{\sin\/}\nolimits\NVar{z}$: sine function and $k$: real or complex modulus Permalink: http://dlmf.nist.gov/19.2.E16 Encodings: TeX, pMML, png See also: Annotations for 19.2(iii)

## §19.2(iv) A Related Function: $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)$

Let $x\in\mathbb{C}\setminus(-\infty,0)$ and $y\in\mathbb{C}\setminus\{0\}$. We define

 19.2.17 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\frac{1}{2}\int_{0}^{\infty}\frac{% \mathrm{d}t}{\sqrt{t+x}(t+y)},$ Defines: $\mathop{R_{C}\/}\nolimits\!\left(\NVar{x},\NVar{y}\right)$: Carlson’s combination of inverse circular and inverse hyperbolic functions Symbols: $\mathrm{d}\NVar{x}$: differential of $x$ and $\int$: integral Referenced by: §19.10(ii) Permalink: http://dlmf.nist.gov/19.2.E17 Encodings: TeX, pMML, png See also: Annotations for 19.2(iv)

where the Cauchy principal value is taken if $y<0$. Formulas involving $\mathop{\Pi\/}\nolimits\!\left(\phi,\alpha^{2},k\right)$ that are customarily different for circular cases, ordinary hyperbolic cases, and (hyperbolic) Cauchy principal values, are united in a single formula by using $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)$.

In (19.2.18)–(19.2.22) the inverse trigonometric and hyperbolic functions assume their principal values (§§4.23(ii) and 4.37(ii)). When $x$ and $y$ are positive, $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)$ is an inverse circular function if $x and an inverse hyperbolic function (or logarithm) if $x>y$:

 19.2.18 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\frac{1}{\sqrt{y-x}}\mathop{% \mathrm{arctan}\/}\nolimits\sqrt{\frac{y-x}{x}}=\frac{1}{\sqrt{y-x}}\mathop{% \mathrm{arccos}\/}\nolimits\sqrt{x/y},$ $0\leq x,
 19.2.19 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\frac{1}{\sqrt{x-y}}\mathop{% \mathrm{arctanh}\/}\nolimits\sqrt{\frac{x-y}{x}}=\frac{1}{\sqrt{x-y}}\mathop{% \ln\/}\nolimits\frac{\sqrt{x}+\sqrt{x-y}}{\sqrt{y}},$ $0.

The Cauchy principal value is hyperbolic:

 19.2.20 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\sqrt{\frac{x}{x-y}}\mathop{R_{C}% \/}\nolimits\!\left(x-y,-y\right)=\frac{1}{\sqrt{x-y}}\mathop{\mathrm{arctanh}% \/}\nolimits\sqrt{\frac{x}{x-y}}=\frac{1}{\sqrt{x-y}}\mathop{\ln\/}\nolimits% \frac{\sqrt{x}+\sqrt{x-y}}{\sqrt{-y}},$ $y<0\leq x$.

For the special cases of $\mathop{R_{C}\/}\nolimits\!\left(x,x\right)$ and $\mathop{R_{C}\/}\nolimits\!\left(0,y\right)$ see (19.6.15).

If the line segment with endpoints $x$ and $y$ lies in $\mathbb{C}\setminus(-\infty,0]$, then

 19.2.21 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\int_{0}^{1}(v^{2}x+(1-v^{2})y)^{-% 1/2}\mathrm{d}v,$
 19.2.22 $\mathop{R_{C}\/}\nolimits\!\left(x,y\right)=\frac{2}{\pi}\int_{0}^{\pi/2}% \mathop{R_{C}\/}\nolimits\!\left(y,x{\mathop{\cos\/}\nolimits^{2}}\theta+y{% \mathop{\sin\/}\nolimits^{2}}\theta\right)\mathrm{d}\theta.$