In mathematics, the principal part has several independent meanings but usually refers to the negative-power portion of the Laurent series of a function.
of a function is the portion of the Laurent series consisting of terms with negative degree.
If the Laurent series has an inner radius of convergence of
despite the Laurent series having an infinite principal part.
Consider the difference between the function differential and the actual increment: The differential dy is sometimes called the principal (linear) part of the function increment Δy.
The term principal part is also used for certain kinds of distributions having a singular support at a single point.