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Concept
Principal value
Summary
In mathematics, specifically complex analysis, the principal values of a multivalued function are the values along one chosen branch of that function, so that it is single-valued. A simple case arises in taking the square root of a positive real number. For example, 4 has two square roots: 2 and −2; of these the positive root, 2, is considered the principal root and is denoted as
Consider the complex logarithm function log z. It is defined as the complex number w such that
Now, for example, say we wish to find log i. This means we want to solve
for w. The value iπ/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 arg i. We can rotate counterclockwise π/2 radians from 1 to reach i initially, but if we rotate further another 2π we reach i again. So, we can conclude that i(π/2 + 2π) is also a solution for log i. It becomes clear that we can add any multiple of 2πi 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
for an integer k, where Arg z is the (principal) argument of z defined to lie in the interval . As the principal argument is unique for a given complex number z, is not included in the interval. Each value of k determines what is known as a branch (or sheet), a single-valued component of the multiple-valued log function.
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.
In general, if f(z) is multiple-valued, the principal branch of f is denoted
such that for z in the domain of f, pv f(z) is single-valued.
Complex valued elementary functions 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.
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