f\left( {x_1 } \right) \approx \left( {x_1 - \frac{d}
{{\sqrt 2 }}} \right) + \frac{d}
{{4\sqrt 2 }}\left( {x - \frac{d}
{{\sqrt 2 }}} \right)^2 + \frac{{d^2 }}
{{16}}\left( {x - \frac{d}
{{\sqrt 2 }}} \right)^3