<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tx_min(tasmax, freq='YS'):
 r"""Lowest max temperature The minimum of daily maximum temperature. Parameters tasmax : xarray.DataArray Maximum daily temperature [℃] or [K] freq : str, optional Resampling frequency Returns ------- xarray.DataArray Minimum of daily maximum temperature. Notes ----- Let :math:`TX_{ij}` be the maximum temperature at day :math:`i` of period :math:`j`. Then the minimum daily maximum temperature for period :math:`j` is: .. math:: TXn_j = min(TX_{ij}) """


    return tasmax.resample(time=freq).min(dim='time', keep_attrs=True)