def f(x, a=0., b=0., c=0.):
    """
    Basic function representation: ax^2 + bx + c
    Parameters
    ----------
    x coordinate on the abscissa axis
    a coefficient of x^2
    b coefficient of x
    c known term

    Returns
    -------
    The function value at the given x coordinate
    """
    return a * x ** 2 + b * x + c