Fig. 4

a–c Possible 500 search directions of the q-G method for a 2-D quadratic function at \(\mathbf {x}^0=(11,11)\) obtained from parameters q generated by a Gaussian distribution with mean \(\mu ^k=\mathbf {x}^0\) and standard deviation \(\sigma ^k\) that decreases along the iterative procedure from \(\sigma ^0=10\) and reduction factor \(\beta =0.99\); d evolution of the standard deviation \(\sigma ^k\) by the expression \(\sigma _{k+1}=\beta \cdot \sigma ^k\) used to generate the 500 line segments represented in a–c. The line segment in gray illustrates the steepest descent direction of the function