What is a Bump Function?
A bump function (sometimes called a test function) is, perhaps not surprisingly, a function with a “bump.” A simple example is the real-valued function
The function is defined on the open interval (-1, 1), otherwise it equals zero.
More precisely, a bump function is a smooth function with compact support. Compact support means that the function is zero outside of a compact set. Bump functions are also usually (i.e., there are exceptions) non-negative everywhere, have outputs no more than 1, and equal 1 on the defined compact set . In a sense, they are the smooth analogues of characteristic functions .
Example of Bump Function Uses
Bump functions have a variety of uses in math, including use as mollifiers (smooth functions with special properties), as smooth cutoff functions, or to create smooth partitions of unity. Specific examples include:
- Creating smooth partitions of unity and to extend locally defined smooth functions to globally defined smooth functions .
- Functions created in Euclidean space can be used to construct bump functions on any smooth manifold (where support is prescribed as “equal to one region” . Generalization to higher dimensions can be done with polar coordinates .
- The Hicks-Henne bump function is used in aerodynamics. For example, to model uncertainties on an airfoil’s geometries.
- Keane’s bump function is a standard benchmark for nonlinear constrained optimization .
Graph created with Desmos.
Bump Function Image: Joshdif|Wikimedia Commons. CC 4.0.
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