The logarithmic integral function (or just the “logarithmic integral”) is a locally summable function on the real line. This special function is used in physics and number theory, most notably in the prime number theorem.
The formula for the logarithmic integral function for positive values of x is
Some versions of the integral have a lower bound of integration of 0 (called the American version) instead of 2 (the European version). The two integrals are related by the formula (x) = Li(x) – Li(2) = Li(x) – 1.045163780 . The European version avoids the problem of a singularity at t = 1; Another way to avoid this problem is to use a set of integrals (where PV is the integral’s Cauchy principal value) :
Applications of the Logarithmic Integral Function
The logarithmic integral function is used primarily in physics and number theory.
In number theory, the function makes an appearance in the prime number theorem. The theorem states that as x tends to infinity, the number of primes less than x can be approximated by
Where li(x) is the logarithmic integral :
 Pardo, J. (2012). Cryptography and Cryptanalysis: Primality, Factoring and Discrete Logarithms. Springer.
 Fisher, B. & Al-Sihery, F. (2016). On the Logarithmic Integral and the Convolution. Mathematica Moravica.
 Hildebrand, A. (2015). Short Course on Asymptotics. Mathematics Summer REU Programs. University of Illinois. Retrieved April 5, 2021 from: https://faculty.math.illinois.edu/~hildebr/lecture-notes/asymptotics.pdf
 Tunstrom, K. Prime Number Theorem.
Logarithmic integral function graph: RicHard-59, CC BY-SA 3.0
Stephanie Glen. "Logarithmic Integral Function" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/logarithmic-integral-function/
Need help with a homework or test question? With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. Your first 30 minutes with a Chegg tutor is free!