# Logarithmic functions (§6.4)

As long as *b* ≠ 1,
the exponential function with base *b* is one-to-one,
so it has an inverse.
A **logarithmic function**
is an inverse of an exponential function.
These two statements mean exactly the same thing:
*b*^{x} = *y*;
- log
_{b} *y* = *x*.

The left-hand side of the latter equation
is the **logarithm**, base *b*, of *y*;
logarithms are particularly useful in many applications of mathematics.
If you don't remember any other values of a logarithmic function,
remember these:

- log
_{b} 1 = 0,
- log
_{b} *b* = 1,
- log
_{b} (1/*b*) = −1.

The domain of a logarithmic function is the set of all positive numbers;
the range is the set of all real numbers.
(A logarithm of a negative number is imaginary.)
If *b* > 1, then the logarithmic function is increasing;
if *b* < 1, then the logarithmic function is decreasing.
There are abbreviations for logarithms with certain special bases:

- lb
*x* = log_{2} *x*;
- lg
*x* = log_{10} *x*;
- ln
*x* = log_{e} *x*,
where e is a special number, about 2.72;
- log
*x*
is the logarithm of *x* with whatever is your favourite base.

The book's favourite base is 10, which I will also use.

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This web page was written between 2011 and 2013 by Toby Bartels,
last edited on 2013 May 9.
Toby reserves no legal rights to it.

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