nr.seed(2021)
import statsmodels.tsa.arima_process as arima
def ARMA_model(ar_coef, ma_coef, start = '1-2011', end = '1-2021'):
    dates = pd.date_range(start = start, end = end, freq = 'M')
    ts = arima.ArmaProcess(ar_coef, ma_coef)
    print('Is the time series stationary? ' + str(ts.isstationary))
    print('Is the time series invertible? ' + str(ts.isinvertible))
    return(pd.Series(ts.generate_sample(120), index = dates))
ts_series_ar2 = ARMA_model(ar_coef = [1, .75, .25], ma_coef = [1])