Exponential integral explained
In mathematics, the exponential integral Ei 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 x, the exponential integral Ei(x) is defined as
The Risch algorithm shows that Ei is not an elementary function. The definition above can be used for positive values of x, 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 0 and [1] Instead of Ei, the following notation is used,[2]
E1(z)=
dt, |{\rmArg}(z)|<\pi
For positive values of x, we have
In general, a branch cut is taken on the negative real axis and E1 can be defined by analytic continuation elsewhere on the complex plane.
For positive values of the real part of
, this can be written
[3] E1(z)=
dt=
du, \Re(z)\ge0.
The behaviour of E1 near the branch cut can be seen by the following relation:[4]
\lim\delta\to0+E1(-x\pmi\delta)=-\operatorname{Ei}(x)\mpi\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
For real or complex arguments off the negative real axis,
can be expressed as
[5] E1(z)=-\gamma-lnz-
(\left|\operatorname{Arg}(z)\right|<\pi)
where
is the
Euler–Mascheroni constant. The sum converges for all complex
, 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
with floating point operations for real
between 0 and 2.5. For
, the result is inaccurate due to
cancellation.
A faster converging series was found by Ramanujan:
{\rmEi}(x)=\gamma+lnx+\exp{(x/2)}
| \lfloor(n-1)/2\rfloor |
\sum | |
| k=0 |
Asymptotic (divergent) series
Unfortunately, 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
.
[6] However, for positive values of x, there is a divergent series approximation that can be obtained by integrating
by parts:
[7]
The relative error of the approximation above is plotted on the figure to the right for various values of
, the number of terms in the truncated sum (
in red,
in pink).
Asymptotics beyond all orders
Using integration by parts, we can obtain an explicit formulaFor any fixed
, the absolute value of the error term
decreases, then increases. The minimum occurs at
, at which point
\verten(z)\vert\leq\sqrt{
}e^. This bound is said to be "asymptotics beyond all orders".
Exponential and logarithmic behavior: bracketing
From the two series suggested in previous subsections, it follows that
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,
can be bracketed by elementary functions as follows:
[8]
e-xln\left(1+
\right)
<E1(x)<e-xln\left(1+
\right)
x>0
The left-hand side of this inequality is shown in the graph to the left in blue; the central part
is shown in black and the right-hand side is shown in red.
Definition by Ein
Both
and
can be written more simply using the
entire function
[9] defined as
\operatorname{Ein}(z)
=
(1-e-t)
=
(note that this is just the alternating series in the above definition of
). Then we have
E1(z)=-\gamma-lnz+{\rmEin}(z)
\left|\operatorname{Arg}(z)\right|<\pi
\operatorname{Ei}(x)=\gamma+ln{x}-\operatorname{Ein}(-x)
x ≠ 0
Relation with other functions
Kummer's equation
is usually solved by the
confluent hypergeometric functions
and
But when
and
that is,
we have
for all
z. A second solution is then given by E
1(−
z). In fact,
E | |
| 1(-z)=-\gamma-i\pi+ | \partial[U(a,1,z)-M(a,1,z)] | \partiala |
|
, 0<{\rmArg}(z)<2\pi
with the derivative evaluated at
Another connexion with the confluent hypergeometric functions is that
E1 is an exponential times the function
U(1,1,
z):
The exponential integral is closely related to the logarithmic integral function li(x) by the formula
\operatorname{li}(ex)=\operatorname{Ei}(x)
for non-zero real values of
.
Generalization
The exponential integral may also be generalized to
which can be written as a special case of the upper incomplete gamma function:[10]
The generalized form is sometimes called the Misra function[11]
, defined as
Many properties of this generalized form can be found in the NIST Digital Library of Mathematical Functions.
Including a logarithm defines the generalized integro-exponential function[12]
The indefinite integral:
\operatorname{Ei}(a ⋅ b)=\iinteadadb
is similar in form to the ordinary
generating function for
, the number of
divisors of
:
Derivatives
The derivatives of the generalised functions
can be calculated by means of the formula
[13] En'(z)=-En-1(z)
(n=1,2,3,\ldots)
Note that the function
is easy to evaluate (making this recursion useful), since it is just
.
[14] Exponential integral of imaginary argument
If
is imaginary, it has a nonnegative real part, so we can use the formula
to get a relation with the
trigonometric integrals
and
:
E1(ix)=i\left[-\tfrac{1}{2}\pi+\operatorname{Si}(x)\right]-\operatorname{Ci}(x)
(x>0)
The real and imaginary parts of
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[15] where
A &= \ln\left [\left (\frac{0.56146}{x}+0.65\right)(1+x)\right] \\B &= x^4e^(2+x)^\end
- The Allen and Hastings approximation [16] where
\textbf & \triangleq [-0.57722, 0.99999, -0.24991, 0.05519, -0.00976, 0.00108]^T \\\textbf & \triangleq[0.26777,8.63476, 18.05902, 8.57333]^T \\\textbf & \triangleq[3.95850, 21.09965, 25.63296, 9.57332]^T \\\textbf_k &\triangleq[x^0,x^1,\dots, x^k]^T\end
- The continued fraction expansion
- The approximation of Barry et al. [17] where:
h &= \frac+\frac \\q &=\fracx^ \\h_ &= \frac \\b &=\sqrt \\G &= e^\end with
being the
Euler–Mascheroni constant.
Inverse function of the Exponential Integral
We can express the Inverse function of the exponential integral in power series form:[18]
where
is the
Ramanujan–Soldner constant and
is
polynomial sequence defined by the following
recurrence relation:
P0(x)=x, Pn+1(x)=x(Pn'(x)-nPn(x)).
For
,
and we have the formula :
Pn(x)=\left.\left(
\right)n-1\left(
.
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[19]
See also
References
- Book: Abramowitz
, Milton
. . Irene Stegun . Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables . Dover . 1964 . New York . 978-0-486-61272-0 . registration., Chapter 5.
- Book: Bender
, Carl M.
. Steven A. Orszag . Advanced mathematical methods for scientists and engineers . McGraw–Hill . 1978 . 978-0-07-004452-4.
- Book: Bleistein
, Norman
. Richard A. Handelsman . Asymptotic Expansions of Integrals . Dover . 1986 . 978-0-486-65082-1.
- 10.1093/qmath/1.1.176 . Ida W. . Busbridge . Quart. J. Math. (Oxford) . 1950 . 1 . 1 . On the integro-exponential function and the evaluation of some integrals involving it . 176–184 . 1950QJMat...1..176B.
- A.. Stankiewicz. Tables of the integro-exponential functions. Acta Astronomica. 18. 289. 1968. 1968AcA....18..289S.
- R. R.. Sharma. Bahman. Zohuri. A general method for an accurate evaluation of exponential integrals E1(x), x>0. J. Comput. Phys.. 25. 2. 199–204. 10.1016/0021-9991(77)90022-5. 1977. 1977JCoPh..25..199S.
- 10.1090/S0025-5718-1983-0701632-1 . K. S.. Kölbig. On the integral exp(-μt)tν-1logmt dt. Math. Comput.. 1983. 171–182. 41. 163. free.
- 10.1090/S0025-5718-1985-0777276-4 . M. S. . Milgram . Mathematics of Computation . The generalized integro-exponential function . 44 . 170 . 1985 . 0777276 . 443 - 458 . 2007964. free .
- Misra. Rama Dhar. 1940. On the Stability of Crystal Lattices. II. Mathematical Proceedings of the Cambridge Philosophical Society. 36. 2. 173. 10.1017/S030500410001714X. Born. M.. 1940PCPS...36..173M . 251097063.
- C.. Chiccoli. S.. Lorenzutta. G.. Maino. On the evaluation of generalized exponential integrals Eν(x). J. Comput. Phys.. 78. 2. 278–287. 1988. 10.1016/0021-9991(88)90050-2. 1988JCoPh..78..278C.
- C.. Chiccoli. S.. Lorenzutta. G.. Maino. Recent results for generalized exponential integrals. Computer Math. Applic.. 19. 5. 21–29. 1990. 10.1016/0898-1221(90)90098-5.
- Allan J.. MacLeod. The efficient computation of some generalised exponential integrals. J. Comput. Appl. Math.. 10.1016/S0377-0427(02)00556-3. 2002. 148. 2. 363–374. 2002JCoAM.148..363M. free.
External links
Notes and References
- Abramowitz and Stegun, p. 228
- Abramowitz and Stegun, p. 228, 5.1.1
- Abramowitz and Stegun, p. 228, 5.1.4 with n = 1
- Abramowitz and Stegun, p. 228, 5.1.7
- Abramowitz and Stegun, p. 229, 5.1.11
- Bleistein and Handelsman, p. 2
- Bleistein and Handelsman, p. 3
- Abramowitz and Stegun, p. 229, 5.1.20
- Abramowitz and Stegun, p. 228, see footnote 3.
- Abramowitz and Stegun, p. 230, 5.1.45
- After Misra (1940), p. 178
- Milgram (1985)
- Abramowitz and Stegun, p. 230, 5.1.26
- Abramowitz and Stegun, p. 229, 5.1.24
- Revisit of Well Function Approximation and An Easy Graphical Curve Matching Technique for Theis' Solution. Ground Water. 2003-05-01. 1745-6584. 387–390. 41. 3. 10.1111/j.1745-6584.2003.tb02608.x. Pham Huy. Giao. 12772832 . 2003GrWat..41..387G . 31982931 .
- Numerical evaluation of exponential integral: Theis well function approximation. Journal of Hydrology. 1998-02-26. 38–51. 205. 1–2. 10.1016/S0022-1694(97)00134-0. Peng-Hsiang. Tseng. Tien-Chang. Lee. 1998JHyd..205...38T .
- Approximation for the exponential integral (Theis well function) . Journal of Hydrology. 2000-01-31. 287–291. 227. 1–4. 10.1016/S0022-1694(99)00184-5. D. A. Barry. J. -Y. Parlange . L. Li. 2000JHyd..227..287B .
- Web site: Inverse function of the Exponential Integral . 2024-04-24 . Mathematics Stack Exchange .
- Book: Nuclear Reactor Theory. 1970. Van Nostrand Reinhold Company. George I. Bell. Samuel Glasstone.