Shekel function

Shekel function is a multidimensional, multimodal, continuous, deterministic function commonly used as a test function for testing optimization techniques.

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

$f(\vec{x}) = \sum_{i = 1}^{m} \tfrac{1}{c_{i} + \sum\limits_{j = 1}^{n} (x_{j} - a_{ji})^2 }$

or, similarly,

$f(x_1,x_2,...,x_{n-1},x_n) = \sum_{i = 1}^{m} \tfrac{1}{c_{i} + \sum\limits_{j = 1}^{n} (x_{j} - a_{ji})^2 }$

A Shekel function in 2 dimensions and with 10 maxima

References

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