List of logarithmic identities


In mathematics, there are many logarithmic identities.

Trivial identities

Cancelling exponentials

Logarithms and exponentials with the same base cancel each other. This is true because logarithms and exponentials are inverse operations.
Both of the above are derived from the following two equations that define a logarithm:
Substituting in the left equation gives, and substituting in the right gives. Finally, replace with.

Using simpler operations

Logarithms can be used to make calculations easier. For example, two numbers can be multiplied just by using a logarithm table and adding. The first three operations below assume, and/or so that and. Derivations also use the log definitions and.
Where,, and are positive real numbers and. Both and are real numbers.
The laws result from canceling exponentials and appropriate law of indices. Starting with the first law:
The law for powers exploits another of the laws of indices:
The law relating to quotients then follows:
Similarly, the root law is derived by rewriting the root as a reciprocal power:

Changing the base

This identity is useful to evaluate logarithms on calculators. For instance, most calculators have buttons for ln and for, but not all calculators have buttons for the logarithm of an arbitrary base.
This formula has several consequences:
where is any permutation of the subscripts 1, ..., n. For example

Summation/subtraction

The following summation/subtraction rule is especially useful in probability theory when one is dealing with a sum of log-probabilities:
Note that in practice and have to be switched on the right hand side of the equations if. Also note that the subtraction identity is not defined if since the logarithm of zero is not defined. Many programming languages have a specific log1p function that calculates without underflow when is small.
More generally:
where are sorted in descending order.

Exponents

A useful identity involving exponents:
or more universally:

Other/Resulting Identities

Inequalities

Based on , and
All are accurate around, but not for large numbers.

Calculus identities

Limits">Limit of a function">Limits

The last limit is often summarized as "logarithms grow more slowly than any power or root of x".

[Derivative]s of logarithmic functions

Where,, and.

Integral definition

[Integral]s of logarithmic functions

To remember higher integrals, it's convenient to define:
Where is the nth Harmonic number.
Then,

Approximating large numbers

The identities of logarithms can be used to approximate large numbers. Note that, where a, b, and c are arbitrary constants. Suppose that one wants to approximate the 44th Mersenne prime,. To get the base-10 logarithm, we would multiply 32,582,657 by, getting. We can then get.
Similarly, factorials can be approximated by summing the logarithms of the terms.

Complex logarithm identities

The complex logarithm is the complex number analogue of the logarithm function. No single valued function on the complex plane can satisfy the normal rules for logarithms. However a multivalued function can be defined which satisfies most of the identities. It is usual to consider this as a function defined on a Riemann surface. A single valued version called the principal value of the logarithm can be defined which is discontinuous on the negative x axis and equals the multivalued version on a single branch cut.

Definitions

The convention will be used here that a capital first letter is used for the principal value of functions and the lower case version refers to the multivalued function. The single valued version of definitions and identities is always given first followed by a separate section for the multiple valued versions.
The multiple valued version of is a set but it is easier to write it without braces and using it in formulas follows obvious rules.
When k is any integer:

Constants

Principal value forms:
Multiple value forms, for any k an integer:

Summation

Principal value forms:
Multiple value forms:

Powers

A complex power of a complex number can have many possible values.
Principal value form:
Multiple value forms:
Where, are any integers: