Shekel function

Shekel function is a multimodal function commonly used as a test function for testing optimization techniques.

The mathematical form of a function in n dimensions with m minima is:

$f(x)=-\sum _{i=1}^{n}{\frac {1}{c_{i}+\sum _{j=1}^{m}(x_{j}-a_{ji})^{2}}}$

Source:

Shekel, J. 1971. "Test Functions for Multimodal Search Techniques." Fifth Annual Princeton Conference on Information Science and Systems.