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Logarithm as Inverse of Exponential
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Logarithms
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Given  and  , with  , the logarithm base  of  , written ](/support/helpjp/helpview.aspx?si=8419/file02290/math39.png) is the exponent to which  needs to be raised to obtain  . That is,  = y](/support/helpjp/helpview.aspx?si=8419/file02290/math45.png) means exactly that  . Thus, the functions ](/support/helpjp/helpview.aspx?si=8419/file02290/math49.png) and  are inverses of each other. The domain of the logarithm base  is all positive numbers. The range of the logarithm base  is all real numbers.
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General Logarithms
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Recall that the domain and range of an invertible function are just the range and domain of its inverse. Thus, the domain of the logarithm base  function is the range of the  function (all positive numbers) and the range of the logarithm base  function is the domain of the  function (all numbers).
Examples:
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 = 3](/support/helpjp/helpview.aspx?si=8419/file02290/math100.png) since 
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 = -3](/support/helpjp/helpview.aspx?si=8419/file02290/math107.png) since 
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The Natural Logarithm Function
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One exponential function is so important in mathematics that it is distinguished by calling it the exponential function. This exponential function is written as  or, particularly when the expression in the exponent is complicated,  . The inverse of this function is just as important in mathematics.
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The Natural Logarithm Function
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The natural logarithm function is the inverse of the exponential function,  , where  . This function is so important in mathematic, science, and engineering that it is given the name "ln": ](/support/helpjp/helpview.aspx?si=8419/file02290/math176.png) . Reading out loud, it is pronounced "lawn of x" or often just "lawn x".
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The graph of the natural logarithm function can be obtained from that of the exponential function by reflection across the line  :
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Exploring the function  with base greater than 1 and between 0 and 1](/support/helpjp/helpview.aspx?si=8419/file02290/math225.png)
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Use the sliders below the graphs to change the values of  , the base of the logarithmic function ](/support/helpjp/helpview.aspx?si=8419/file02290/math233.png) and its corresponding exponential function  . For the graph on the left, the base is a number greater than 1. For the graph on the right, the base is a number between 0 and 1. Note that there is no logarithmic function with base  . Do you see why not?
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