eries.

Convert a z series to the equivalent Chebyshev series. The result is
never an empty array. The dtype of the return is the same as that of
the input. No checks are run on the arguments as this routine is for
internal use.

Parameters
----------
zs : 1-D ndarray
    Odd length symmetric z-series, ordered from  low to high.

Returns
-------
c : 1-D ndarray
    Chebyshev coefficients, ordered from  low to high.

r