The standard fminsearch function uses an unreliable and faulty method for setting the initial x step, e.g. as illustrated by the case fminsearch(@(x) (x-1)^2,1e-3). (The return value is close to zero.) fminsearch_ provides user control of the x step, and uses a more reliable default, e.g. fminsearch_(@(x) (x-1)^2,1e-3) returns 1.