Exponential integral

Plot of the exponential integral function Template:Math for complex Template:Mvar

In mathematics, the exponential integral Template:Tmath is a special function on the complex plane. It is defined as one particular definite integral of the ratio between an exponential function and its argument.

Definitions

For real non-zero values of Template:Tmath, the exponential integral Template:Tmath is defined as $$\mathrm{Ei}(x)=-{\int }_{-x}^{\mathrm{\infty }}\frac{e^{-t}}{t}dt={\int }_{-\mathrm{\infty }}^x\frac{e^t}{t}dt.$$

The Risch algorithm shows that Template:Tmath is not an elementary function. The definition above can be used for positive values of Template:Tmath, but the integral has to be understood in terms of the Cauchy principal value due to the singularity of the integrand at zero.

For complex values of the argument, the definition becomes ambiguous due to branch points at Template:Tmath and Template:Tmath.Template:Sfn Instead of Template:Tmath, the following notation is used,Template:Sfn $$E_1(z)={\int }_z^{\mathrm{\infty }}\frac{e^{-t}}{t}dt,|\mathrm{Arg}(z)|<\pi$$

Plot of the exponential integral function Template:Math for complex Template:Mvar

For positive values of Template:Tmath, we have Template:Tmath.

In general, a branch cut is taken on the negative real axis and Template:Tmath can be defined by analytic continuation elsewhere on the complex plane.

For positive values of the real part of Template:Tmath, this can be writtenTemplate:Sfn $$E_1(z)={\int }_1^{\mathrm{\infty }}\frac{e^{-tz}}{t}dt={\int }_0^1\frac{e^{-z/u}}{u}du,\mathrm{\Re }(z)\ge 0.$$

The behaviour of Template:Tmath near the branch cut can be seen by the following relation:Template:Sfn $$\underset{\delta \to 0+}{lim}{E}_1(-x\pm i\delta )=-\mathrm{Ei}(x)\mp i\pi ,x>0.$$

Properties

Several properties of the exponential integral below, in certain cases, allow one to avoid its explicit evaluation through the definition above.

Convergent series

Plot of Template:Math function (top) and Template:Math function (bottom).

For real or complex arguments off the negative real axis, $E_1(z)$ can be expressed asTemplate:Sfn $$E_1(z)=-\gamma -\ln z-\sum _{k=1}^{\mathrm{\infty }}\frac{(-z{)}^k}{kk!}(|\mathrm{Arg}(z)|<\pi )$$ where $\gamma$ is the Euler–Mascheroni constant. The sum converges for all complex Template:Tmath, and we take the usual value of the complex logarithm having a branch cut along the negative real axis.

This formula can be used to compute $E_1(x)$ with floating point operations for real $x$ between Template:Tmath and Template:Tmath. For Template:Tmath, the result is inaccurate due to cancellation.

A faster converging series was found by Ramanujan:Template:Sfn $$\mathrm{E}\mathrm{i}(x)=\gamma +\ln x+\exp (x/2)\sum _{n=1}^{\mathrm{\infty }}\frac{(-1{)}^{n-1}{x}^n}{n!2^{n-1}}\sum _{k=0}^{\lfloor (n-1)/2\rfloor }\frac{1}{2k+1}$$

Asymptotic (divergent) series

Relative error of the asymptotic approximation for different number Template:Mvar of terms in the truncated sum

The convergence of the series above is slow for arguments of larger modulus. For example, more than 40 terms are required to get an answer correct to three significant figures for Template:Tmath.Template:Sfn However, for positive values of Template:Tmath, there is a divergent series approximation that can be obtained by integrating $x{e}^x{E}_1(x)$ by parts:Template:Sfn $$E_1(x)=\frac{\exp (-x)}{x}(\sum _{n=0}^{N-1}\frac{n!}{(-x{)}^n}+O(N!x^{-N}))$$ The relative error of the approximation above is plotted on the figure to the right for various values of Template:Tmath, the number of terms in the truncated sum (Template:Tmath in red, $N=5$ in pink).

Asymptotics beyond all orders

Normalized exponential integral. The value plotted is Template:Math. The values of Template:Mvar are written above the corresponding point. The horizontal spacing is according to Template:Math. The graph is extended "beyond infinity" a little on both the right and the left to show how the normalized function behaves when Template:Math is small. (The horizontal spacing for these points corresponds to angles whose tangent is Template:Mvar.)

Using integration by parts, we can obtain an explicit formula $$\begin{array}{cc}\mathrm{Ei}(z)& =\frac{e^z}{z}(\sum _{k=0}^n\frac{k!}{z^k}+e_n(z)),\\ e_n(z)& \equiv (n+1)!z{e}^{-z}{\int }_{-\mathrm{\infty }}^z\frac{e^t}{t^{n+2}}dt.\end{array}$$ For any fixed Template:Tmath, the absolute value of the error term $|e_n(z)|$ decreases, then increases. The minimum occurs at Template:Tmath, at which point Template:Tmath. This bound is said to be "asymptotics beyond all orders".

Exponential and logarithmic behavior: bracketing

Bracketing of Template:Math by elementary functions

From the two series suggested in previous subsections, it follows that $E_1$ behaves like a negative exponential for large values of the argument and like a logarithm for small values. For positive real values of the argument, $E_1$ can be bracketed by elementary functions as follows:Template:Sfn $$\frac{1}{2}{e}^{-x}\ln (1+\frac{2}{x})<E_1(x)<e^{-x}\ln (1+\frac{1}{x})x>0$$

The left-hand side of this inequality is shown in the graph to the left in blue; the central part $E_1(x)$ is shown in black and the right-hand side is shown in red.

Definition by Ein

Both $\mathrm{Ei}$ and $E_1$ can be written more simply using the entire function Template:TmathTemplate:Sfn defined as $$\mathrm{Ein}(z)={\int }_0^z(1-e^{-t})\frac{dt}{t}=\sum _{k=1}^{\mathrm{\infty }}\frac{(-1{)}^{k+1}{z}^k}{kk!}$$ (note that this is just the alternating series in the above definition of Template:Tmath). Then we have $$\begin{array}{cccc}{E}_1(z)& =-\gamma -\ln z+\mathrm{Ein}(z),& & |\mathrm{Arg}(z)|<\pi ;\\ \mathrm{Ei}(x)& =\phantom{-}\gamma +\ln x-\mathrm{E}\mathrm{i}\mathrm{n}(-x),& & x\ne 0.\end{array}$$ The function $\mathrm{Ein}$ is related to the exponential generating function of the harmonic numbers: $$\mathrm{Ein}(z)=e^{-z}\sum _{n=1}^{\mathrm{\infty }}\frac{z^n}{n!}{H}_n$$

Relation with other functions

Kummer's equation $$z\frac{d^{2}w}{d{z}^2}+(b-z)\frac{dw}{dz}-aw=0$$ is usually solved by the confluent hypergeometric functions $M(a,b,z)$ and Template:Tmath. But when $a=0$ and Template:Tmath, that is, $$z\frac{d^{2}w}{d{z}^2}+(1-z)\frac{dw}{dz}=0$$ we have $$M(0,1,z)=U(0,1,z)=1$$ for all Template:Tmath. A second solution is then given by Template:Tmath. In fact, $$E_1(-z)=-\gamma -i\pi +\frac{\partial [U(a,1,z)-M(a,1,z)]}{\partial a},0<\mathrm{Arg}(z)<2\pi$$ with the derivative evaluated at Template:Tmath. Another connexion with the confluent hypergeometric functions is that Template:Tmath is an exponential times the function Template:Tmath: $$E_1(z)=e^{-z}U(1,1,z)$$

The exponential integral is closely related to the logarithmic integral function Template:Tmath by the formula $$\mathrm{li}(e^x)=\mathrm{Ei}(x)$$ for non-zero real values of Template:Tmath.

The series expansion of the exponential integral immediately gives rise to an expression in terms of the generalized hypergeometric function Template:Tmath: $$\mathrm{Ei}(x)=x{}_2{F}_2(1,1;2,2;x)+\ln x+\gamma .$$

Generalization

The exponential integral may also be generalized to $$E_n(x)={\int }_1^{\mathrm{\infty }}\frac{e^{-xt}}{t^n}dt,$$ which can be written as a special case of the upper incomplete gamma function:Template:Sfn $$E_n(x)=x^{n-1}\mathrm{\Gamma }(1-n,x).$$

The generalized form is sometimes called the Misra function Template:Tmath,Template:EfnTemplate:Efn defined as $${\phi }_m(x)=E_{-m}(x).$$

Including a logarithm defines the generalized integro-exponential functionTemplate:Sfn $$E_s^j(z)=\frac{1}{\mathrm{\Gamma }(j+1)}{\int }_1^{\mathrm{\infty }}{(\log t)}^j\frac{e^{-zt}}{t^s}dt.$$

Derivatives

The derivatives of the generalised functions $E_n$ can be calculated by means of the formulaTemplate:Sfn $${E_n}^{\prime }(z)=-E_{n-1}(z)(n=1,2,3,\dots )$$ Note that the function $E_0$ is easy to evaluate (making this recursion useful), since it is just Template:Tmath.Template:Sfn

Exponential integral of imaginary argument

Template:Math against Template:Mvar; real part black, imaginary part red.

If $z$ is imaginary, it has a nonnegative real part, so we can use the formula $$E_1(z)={\int }_1^{\mathrm{\infty }}\frac{e^{-tz}}{t}dt$$ to get a relation with the trigonometric integrals $\mathrm{Si}$ and Template:Tmath: $$E_1(ix)=i[-\frac{1}{2}\pi +\mathrm{Si}(x)]-\mathrm{Ci}(x)(x>0)$$ The real and imaginary parts of $E_1(ix)$ are plotted in the figure to the right with black and red curves.

Approximations

There have been a number of approximations for the exponential integral function. These include:

• The Swamee and Ohija approximation $$E_1(x)={(A^{-7.7}+B)}^{-0.13},$$ where $$\begin{array}{cc}A& =\ln [(\frac{0.56146}{x}+0.65)(1+x)]\\ B& =x^4{e}^{7.7x}(2+x{)}^{3.7}\end{array}$$
• The Allen and Hastings approximation $$E_1(x)=\{\begin{array}{cc}-\ln x+{\mathbf{\text{𝐚}}}^{𝖳}{\mathbf{\text{𝐱}}}_5,& x\le 1\\ \frac{e^{-x}}{x}\frac{{\mathbf{\text{𝐛}}}^{𝖳}{\mathbf{\text{𝐱}}}_3}{{\mathbf{\text{𝐜}}}^{𝖳}{\mathbf{\text{𝐱}}}_3},& x\ge 1\end{array}$$ where $$\begin{array}{cc}\mathbf{\text{𝐚}}& \triangleq [-0.57722,0.99999,-0.24991,0.05519,-0.00976,0.00108{]}^{𝖳}\\ \mathbf{\text{𝐛}}& \triangleq [0.26777,8.63476,18.05902,8.57333{]}^{𝖳}\\ \mathbf{\text{𝐜}}& \triangleq [3.95850,21.09965,25.63296,9.57332{]}^{𝖳}\\ {\mathbf{\text{𝐱}}}_k& \triangleq [x^0,x^1,\dots ,x^k{]}^{𝖳}\end{array}$$
• The continued fraction expansion $$E_1(x)=\frac{e^{-x}}{x+\frac{1}{1+\frac{1}{x+\frac{2}{1+\frac{2}{x+\frac{3}{\ddots }}}}}}.$$
• The approximation of Barry et al. $$E_1(x)=\frac{e^{-x}}{G+(1-G)e^{-\frac{x}{1-G}}}\ln [1+\frac{G}{x}-\frac{1-G}{(h+bx{)}^2}],$$ where: $$\begin{array}{cc}h& =\frac{1}{1+x\sqrt{x}}+\frac{h_{\mathrm{\infty }}q}{1+q}\\ q& =\frac{20}{47}{x}^{\sqrt{\frac{31}{26}}}\\ h_{\mathrm{\infty }}& =\frac{(1-G)(G^2-6G+12)}{3G(2-G{)}^{2}b}\\ b& =\sqrt{\frac{2(1-G)}{G(2-G)}}\\ G& =e^{-\gamma }\end{array}$$ with $\gamma$ being the Euler–Mascheroni constant.

Inverse function of the exponential integral

Template:User-generated We can express the Inverse function of the exponential integral in power series form: $$\mathrm{\forall }|x|<\frac{\mu }{\ln (\mu )},{\mathrm{Ei}}^{-1}(x)=\sum _{n=0}^{\mathrm{\infty }}\frac{x^n}{n!}\frac{P_n(\ln (\mu ))}{{\mu }^n}$$ where $\mu$ is the Ramanujan–Soldner constant and $(P_n)$ is polynomial sequence defined by the following recurrence relation: $$P_0(x)=x,P_{n+1}(x)=x({P_n}^{\prime }(x)-n{P}_n(x)).$$

For Template:Tmath, $\mathrm{deg}{P}_n=n$ and we have the formula: $$P_n(x)={{\left(\frac{d}{dt}\right)}^{n-1}{\left(\frac{t{e}^x}{\mathrm{Ei}(t+x)-\mathrm{Ei}(x)}\right)}^n|}_{t=0}.$$

Applications

• Time-dependent heat transfer
• Nonequilibrium groundwater flow in the Theis solution (called a well function)
• Radiative transfer in stellar and planetary atmospheres
• Radial diffusivity equation for transient or unsteady state flow with line sources and sinks
• Solutions to the neutron transport equation in simplified 1-D geometries
• Solutions to the Trachenko–Zaccone nonlinear differential equation for the stretched exponential function in the relaxation of amorphous solids and glass transition

See also

• Goodwin–Staton integral
• Bickley–Naylor functions

References

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• [DLMF 8.19] Template:Dlmf
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External links

• Template:Springer
• NIST documentation on the Generalized Exponential Integral
• Template:MathWorld
• Template:MathWorld
• Template:WolframFunctionsSite
• Exponential, Logarithmic, Sine, and Cosine Integrals in DLMF.