Logarithmic Integral Function

In mathematics, the logarithmic integral function or integral logarithm li(x) is a non-elementary function defined for all positive real numbers x≠ 1 by the definite integral:
{\rm li} (x) = \int_{0}^{x} \frac{dt}{\ln (t)} \; .
Here, ln denotes the natural logarithm. The function 1/ln (t) has a singularity at t = 1, and the integral for x > 1 has to be interpreted as a Cauchy principal value:
{\rm li} (x) = \lim_{\varepsilon \to 0} \left( \int_{0}^{1-\varepsilon} \frac{dt}{\ln (t)} + \int_{1+\varepsilon}^{x} \frac{dt}{\ln (t)} \right) \; .
Sometimes instead of li the offset logarithmic integral is used, defined as {\rm Li}(x) = {\rm li}(x) - {\rm li}(2). This is often used in number theoretic applications. Neither function should be confused with the logarithmic integral whose definition is
\int_{-\infty}^\infty \frac{M(t)}{1+t^2}dt.
The growth behavior of this function for x → ∞ is
{\rm li} (x) = \Theta \left( {x\over \ln (x)} \right) \; .
(see big O notation). The logarithmic integral finds application in many areas, in particular it is used is in estimates of prime number densities, such as the prime number theorem:
π(x) ~ li(x) ~ Li(x)
where π(x) denotes the number of primes smaller than or equal to x. The function li(x) is related to the exponential integral Ei(x) via the equation
li(x) = Ei (ln (x))    for all positive real x ≠ 1.
This leads to series expansions of li(x), for instance:
{\rm li} (e^{u}) = \gamma + \ln \left| (u) \right| + \sum_{n=1}^{\infty} {u^{n}\over n \cdot n!} \quad {\rm for} \; u \ne 0 \; ,
where γ ≈ 0.57721 56649 01532 ... is the Euler-Mascheroni gamma constant. The function li(x) has a single positive zero; it occurs at x ≈ 1.45136 92348 ...; this number is known as the Ramanujan-Soldner constant.

 

<< PreviousWord BrowserNext >>
mac os x server
aibo
nice
the register
edmund ii of england
princess superstar
friedrich engels
unknown dj
edwy of england
montpelier
ip over avian carriers
alphonse de polignac
phosgene
famous non quotation
darius i of persia
darius ii of persia
darius iii of persia
henry john stephen smith
easter bunny
doe maar
baby boomer
ivan novikoff
airborne warning and control system
951 gaspra
nederpop
anglo chinese college
incandescent light bulb
vesta
minor planet
cmo
shirley bassey
jeffrey moussaieff masson
1905 world series
louis the german
ivor novello
means of production
conrad celtes
green economics
emperor shoko of japan
flapping router
navy list
survivors of the valdez oil spill
misanthropy
pope paul ii