def sp_in(c, x):
    """ Spectral index in freq-flux space """
    import numpy as N
    order = len(c) - 1
    if order == 1:
        y = c[0] * N.power(x, c[1])
    elif order == 2:
        y = c[0] * N.power(x, c[1]) * N.power(x, c[2] * N.log(x))
    else:
        print('Not yet implemented')
    return y
