def derive
      dérivé = Polynomial.new

      if @coef.size > 0
        (1..(@coef.size)).times do |coef|
          dérivé.coef[coef - 1] = dérivé[coef] * coef
        end
      end

      return dérivé
    end
def derive
      dérivé = Polynomial.new

      if @coef.size > 0
        (1..(@coef.size)).times do |coef|
          dérivé.coef[coef - 1] = dérivé[coef] * coef
        end
      end

      return dérivé
    end
Initialize the polynominal function
 Its coeficients are 1, 2, 3, 4 ... with '1'x⁰ + '2'x¹ + '3'x² ... = y
 Each coeficients has an associated value (exemple : 2 => 1 correspond to 1x²)
 exemple :
 a = [] ; a[2] = 1 ; #correspond to 0 + 0x + 2x³

 == Parameters:
 coef::
   coef is an array which have several keys 0, 1, 2,... which correspond to the coeficients
 verb:: default is false
   It will display more information if turned on when to_s.
   It's a integer, it must be in the list :
   - 0 : returns ""
   - 1 : "y = equation"
   - 2 : "f(x) = equation" (default value)
calculate the derivated function of the current polynomial
 == Returns:
 Polynomial (the derivated function)