Principal value

The principal branch of a multivariate function is one of these maximal branches that is selected once for all. Typically, the principal branch is the branch that takes a real value for small positive values of the variable.

A principal value is the value at a point of the function defined by the principal branch. In many cases, the principal value at a point of a multivalued function is distinguished from the other values by being the one whose argument has the smallest absolute value, and, when there are two such values, the one with positive real part.

A simple example is given by the square root function: every nonzero complex number has two square roots. The principal value of the square root of a positive real number is the positive square root denoted ⁠${\displaystyle \sqrt{x}}$⁠. The principal square root of a non real complex number is the one with an argument in the interval ⁠${\displaystyle (-\pi /2,\pi /2)}$⁠, and the principal square root of a negative real number ⁠${\displaystyle -x}$⁠ is ⁠${\displaystyle i\sqrt{x}}$⁠.

Motivation

Consider the complex logarithm function log z. It is defined as the complex number w such that

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

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

${\displaystyle e^w=i}$

for ${\displaystyle w}$. The value ${\displaystyle i\pi /2}$ is a solution.

However, there are other solutions, which is evidenced by considering the position of i in the complex plane and in particular its argument ${\displaystyle \arg i}$. We can rotate counterclockwise ${\displaystyle \pi /2}$ radians from 1 to reach i initially, but if we rotate further another ${\displaystyle 2\pi }$ we reach i again. So, we can conclude that ${\displaystyle i(\pi /2+2\pi )}$ is also a solution for log i. It becomes clear that we can add any multiple of ${\displaystyle 2\pi }$ 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

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

for an integer k, where Arg z is the (principal) argument of z defined to lie in the interval ${\displaystyle (-\pi ,\pi ]}$. Each value of k determines what is known as a branch (or sheet), a single-valued component of the multiple-valued log function. When the focus is on a single branch, sometimes a branch cut is used; in this case removing the non-positive real numbers from the domain of the function and eliminating ${\displaystyle \pi }$ as a possible value for Arg z. With this branch cut, the single-branch function is continuous and analytic 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.