Principal value

<h2 id="motivation">Motivation</h2>
Consider the <a href="/facts/Complex_logarithm/X2U0YQuG">complex logarithm</a> function log z. It is defined as the <a href="/facts/Complex_number/2w7mqI7e">complex number</a> w such that

e
          
            w
          
        
        =
        z
        .
      
    
    {\displaystyle e^{w}=z.}

Now, for example, say we wish to find log i. This means we want to solve

e
          
            w
          
        
        =
        i
      
    
    {\displaystyle e^{w}=i}

for 
 
 
 
 w
 
 
 {\displaystyle w}
 
. The value 
 
 
 
 i
 π
 
 /
 
 2
 
 
 {\displaystyle i\pi /2}
 
 is a solution.
However, there are other solutions, which is evidenced by considering the position of i in the <a href="/facts/Complex_plane/vE0QrxJp">complex plane</a> and in particular its <a href="/facts/Argument_(complex_analysis)/JSjfxb5U">argument</a> 
 
 
 
 arg
 ⁡
 i
 
 
 {\displaystyle \arg i}
 
. We can rotate counterclockwise 
 
 
 
 π
 
 /
 
 2
 
 
 {\displaystyle \pi /2}
 
 radians from 1 to reach i initially, but if we rotate further another 
 
 
 
 2
 π
 
 
 {\displaystyle 2\pi }
 
 we reach i again. So, we can conclude that 
 
 
 
 i
 (
 π
 
 /
 
 2
 +
 2
 π
 )
 
 
 {\displaystyle i(\pi /2+2\pi )}
 
 is also a solution for log i. It becomes clear that we can add any multiple of 
 
 
 
 2
 π
 
 
 {\displaystyle 2\pi }
 
 to our initial solution to obtain all values for log i.
But this has a consequence that may be surprising in comparison of real valued functions: log i does not have one definite value. For log z, we have

log
        ⁡
        
          z
        
        =
        ln
        ⁡
        
          
            |
          
          z
          
            |
          
        
        +
        i
        
          (
          
            
              a
              r
              g
            
             
            z
          
          )
        
        =
        ln
        ⁡
        
          
            |
          
          z
          
            |
          
        
        +
        i
        
          (
          
            
              A
              r
              g
            
             
            z
            +
            2
            π
            k
          
          )
        
      
    
    {\displaystyle \log {z}=\ln {|z|}+i\left(\mathrm {arg} \ z\right)=\ln {|z|}+i\left(\mathrm {Arg} \ z+2\pi k\right)}

for an <a href="/facts/Integer/PUwwrawx">integer</a> k, where Arg z is the (principal) argument of z defined to lie in the <a href="/facts/Interval_(mathematics)/e9se3vTe">interval</a> 
 
 
 
 (
 −
 π
 ,
  
 π
 ]
 
 
 {\displaystyle (-\pi ,\ \pi ]}
 
. Each value of k determines what is known as a <a href="/facts/Branch_(mathematical_analysis)/XrN2gNon">branch</a> (or sheet), a single-valued component of the multiple-valued log function. When the focus is on a single branch, sometimes a <a href="/facts/Branch_cut/lJ7Y6Iht">branch cut</a> is used; in this case removing the non-positive real numbers from the domain of the function and eliminating 
 
 
 
 π
 
 
 {\displaystyle \pi }
 
 as a possible value for Arg z. With this branch cut, the single-branch function is <a href="/facts/Continuous_function/sKbl02pB">continuous</a> and <a href="/facts/Analytic_function/JhL3z8FP">analytic</a> everywhere in its domain.
The branch corresponding to k = 0 is known as the principal branch, and along this branch, the values the function takes are known as the principal values.

<h2 id="general-case">General case</h2>
In general, if f(z) is multiple-valued, the principal branch of f is denoted

p
          v
        
        
        f
        (
        z
        )
      
    
    {\displaystyle \mathrm {pv} \,f(z)}

such that for z in the <a href="/facts/Domain_of_a_function/FQvoeUpX">domain</a> of f, pv f(z) is single-valued.

<h3>Principal values of standard functions</h3>
Complex valued <a href="/facts/List_of_mathematical_functions/bua9ERtP">elementary functions</a> can be multiple-valued over some domains. The principal value of some of these functions can be obtained by decomposing the function into simpler ones whereby the principal value of the simple functions are straightforward to obtain.

<h4>Logarithm function</h4>
We have examined the <a href="/facts/Logarithm_function/QeXaV97q">logarithm function</a> above, i.e.,

log
        ⁡
        
          z
        
        =
        ln
        ⁡
        
          
            |
          
          z
          
            |
          
        
        +
        i
        
          (
          
            
              a
              r
              g
            
             
            z
          
          )
        
        .
      
    
    {\displaystyle \log {z}=\ln {|z|}+i\left(\mathrm {arg} \ z\right).}

Now, arg z is intrinsically multivalued. One often defines the argument of some complex number to be between 
 
 
 
 −
 π
 
 
 {\displaystyle -\pi }
 
 (exclusive) and 
 
 
 
 π
 
 
 {\displaystyle \pi }
 
 (inclusive), so we take this to be the principal value of the argument, and we write the argument function on this branch Arg z (with the leading capital A). Using Arg z instead of arg z, we obtain the principal value of the logarithm, and we write<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a>

p
          v
        
        log
        ⁡
        
          z
        
        =
        
          L
          o
          g
        
        
        z
        =
        ln
        ⁡
        
          
            |
          
          z
          
            |
          
        
        +
        i
        
          (
          
            
              A
              r
              g
            
            
            z
          
          )
        
        .
      
    
    {\displaystyle \mathrm {pv} \log {z}=\mathrm {Log} \,z=\ln {|z|}+i\left(\mathrm {Arg} \,z\right).}

<h4>Square root</h4>
For a complex number 
 
 
 
 z
 =
 r
 
 e
 
 i
 ϕ
 
 
 
 
 
 {\displaystyle z=re^{i\phi }\,}
 
 the principal value of the <a href="/facts/Square_root/AfrzfBdQ">square root</a> is:

p
          v
        
        
          
            z
          
        
        =
        exp
        ⁡
        
          (
          
            
              
                
                  p
                  v
                
                log
                ⁡
                z
              
              2
            
          
          )
        
        =
        
          
            r
          
        
        
        
          e
          
            i
            ϕ
            
              /
            
            2
          
        
      
    
    {\displaystyle \mathrm {pv} {\sqrt {z}}=\exp \left({\frac {\mathrm {pv} \log z}{2}}\right)={\sqrt {r}}\,e^{i\phi /2}}

with <a href="/facts/Arg_(mathematics)/JSjfxb5U">argument</a> 
 
 
 
 −
 π
 <
 ϕ
 ≤
 π
 .
 
 
 {\displaystyle -\pi <\phi \leq \pi .}
 
 Sometimes a branch cut is introduced so that negative real numbers are not in the domain of the square root function and eliminating the possibility that 
 
 
 
 ϕ
 =
 π
 .
 
 
 {\displaystyle \phi =\pi .}

<h4>Inverse trigonometric and inverse hyperbolic functions</h4>
Inverse trigonometric functions (arcsin, arccos, arctan, etc.) and inverse hyperbolic functions (arsinh, arcosh, artanh, etc.) can be defined in terms of logarithms and their principal values can be defined in terms of the principal values of the logarithm.

<h4>Complex argument</h4>

The principal value of <a href="/facts/Arg_(mathematics)/JSjfxb5U">complex number argument</a> measured in <a href="/facts/Radian/Srd38Sxa">radians</a> can be defined as:

<ul><li>values in the range 
 
 
 
 [
 0
 ,
 2
 π
 )
 
 
 {\displaystyle [0,2\pi )}
 
</li>
<li>values in the range 
 
 
 
 (
 −
 π
 ,
 π
 ]
 
 
 {\displaystyle (-\pi ,\pi ]}
 
</li></ul>
For example, many computing systems include an <a href="/facts/Atan2/0JDnUYfP">atan2(y, x)</a> function. The value of atan2(imaginary_part(z), real_part(z)) will be in the interval 
 
 
 
 (
 −
 π
 ,
 π
 ]
 .
 
 
 {\displaystyle (-\pi ,\pi ].}
 
 In comparison, atan y/x is typically in 
 
 
 
 (
 
 
 
 
 −
 π
 
 2
 
 
 
 ,
 
 
 
 π
 2
 
 
 
 ]
 .
 
 
 {\displaystyle ({\tfrac {-\pi }{2}},{\tfrac {\pi }{2}}].}

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Principal_branch/XrN2gNon">Principal branch</a></li>
<li><a href="/facts/Branch_point/lJ7Y6Iht">Branch point</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Zill, Dennis; Shanahan, Patrick (2009). A First Course in Complex Analysis with Applications. Jones & Bartlett Learning. p. 166. ISBN 978-0-7637-5772-4. <a href="978-0-7637-5772-4" target="_blank">978-0-7637-5772-4</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
</ol>

Principal value open-in-new

Principal value