def constant_sd(data, k=1):
    """method to get anomalies as dates (DatetimeIndex) (dtype=datetime64[ns])
    - constant mean mu
    - constand std sigma
    - data points xn
    - anomalies definition: xn - mu > k*std

    - maximum ten anomalies (10 'biggest' anomalies)

    :argument data: dataframe with one column (interest) and pandas dates as index
    :argument k: float

    :returns anomalies as DatetimeIndex sorted(dtype=datetime64[ns])
    """
    mean = data.mean()[0]
    std = data.std()[0]
    tmp = data - mean - k * std
    return tmp[tmp.iloc[:, (0)] > 0].sort_values(by=tmp.columns[0],
        ascending=False)[0:10].index.sort_values()
