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Find a real root in the given interval.
Despite the bracketing condition, the root returned by
LaguerreSolver.ComplexSolver#solve(Complex[],Complex) may
not be a real zero inside
[min, max].
For example, p(x) = x3 + 1,
with
min = -2,
max = 2,
initial = 0.
When it occurs, this code calls
LaguerreSolver.ComplexSolver#solveAll(Complex[],Complex)in order to obtain all roots and picks up one real root.