code: def test_center(raw):
      obj = Series(np.random.default_rng(2).standard_normal(50))
      obj[:10] = np.nan
      obj[-10:] = np.nan
  
      result = obj.rolling(20, min_periods=15, center=True).apply(
          f, raw=raw
      )
      expected = (
          concat([obj, Series([np.nan] * 9)])
          .rolling(20, min_periods=15)
          .apply(f, raw=raw)
          .iloc[9:]
          .reset_index(drop=True)
      )
      tm.assert_series_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, Index, MultiIndex, Series, Timestamp, concat, date_range, isna, notna
  import pandas._testing
  from pandas.tseries import offsets
label_desc: Use image node_id 13 for calling a global function with example usage: test_center(raw) without return types