  minimum number of digits necessary to represent the value exactly.
    </p>
    <p>
      See <a href='#Errors'>Errors</a> for the treatment of other
      non-integer or out of range <code>decimal_places</code> values.
    </p>
    <pre>
x = 45.6
y = new BigNumber(x)
x.toExponential()         // '4.56e+1'
y.toExponential()         // '4.56e+1'
x.toExponential(0)        // '5e+1'
y.toE(0)                  // '5e+1'
x.toExponential(1)        // '4.6e+1'
y.toE(1)                  // '4.6e+1'
x.toExponential