Minimizes a (given)
function of 2 variables by successive adaptation of a quadratic function,
in the bivariate case, thus using 6 points
to compute the 6 coefficients involved.
The base function is
z = p1 arctan(p2 + x²) +
p3 (x² + y²), with known minimum
at the origin.
The graph shows the range, the difference
from the best (smallest) to the worst (largest) function values
vs. the iteration number.
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