def linear_regression(x):
    x = np.asarray(x, dtype=np.float)
    n_eval = x.shape[0]
    f = np.hstack([np.ones([n_eval, 1]), x])
    return f
First order polynomial (linear, p = n+1) regression model.    
dataset/ETHPy150Open OpenMDAO/OpenMDAO-Framework/openmdao.lib/src/openmdao/lib/surrogatemodels/multifi_cokriging_surrogate.py/linear_regression