Approximate integral for exp(-x)/x

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SUMMARY

The discussion focuses on evaluating the integral \(\int_{x_0}^{\infty} \frac{\exp(-x)}{x}\, dx\) for \(x_0 \ll 1\). Participants reference the exponential integral function, \(Ei(x)\), which is defined as \(Ei(x) = \int_{-\infty}^{x} \frac{e^t}{t} dt\). Prudnikov's formula, \(\int_{x}^{\infty} \frac{e^{-ax}}{x} dx = -Ei(-ax)\) for \(a > 0\), is highlighted as a key analytical expression. The discussion also mentions the series expansion of \(Ei(x)\) and its convergence properties.

PREREQUISITES
  • Understanding of integral calculus, specifically improper integrals.
  • Familiarity with the exponential integral function \(Ei(x)\).
  • Knowledge of series expansions and convergence criteria.
  • Basic concepts of asymptotic analysis for small parameters.
NEXT STEPS
  • Study the properties and applications of the exponential integral function \(Ei(x)\).
  • Learn about asymptotic expansions for integrals with small parameters.
  • Explore numerical methods for evaluating improper integrals.
  • Investigate the convergence of series expansions in mathematical analysis.
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Mathematicians, physicists, and engineers involved in analytical methods, particularly those working with integrals and series in applied mathematics.

Irid
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I need to evaluate this integral
\int_{x_0}^{\infty} \frac{\exp(-x)}{x}\, dx
for
x_0 \ll 1

Is there any cute approximate analytical expression? All I get are diverging series all over the place. The function is obviously finite...
 
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Hi,

Prudnikov: Integrals and series

∫(x,∞) e^(-ax)/x dx = -Ei(-ax) a>0

Ei(x) = ∫(-∞,x) e^t/t dt is the exponential integral



Bronstejn

Ei(x) = ∫(-∞,x) e^t/dt = C + ln|x| + x + x*x/(2*2!) + ... + x^n/(n*n!) + ...

C = 0.577215665

kamke
 
Last edited:

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