code: def test_is_monotonic_decreasing(in_vals, out_vals):
      # GH 17015
      source_dict = {
          "A": [
              "1",
              "2",
              "3",
              "4",
              "5",
              "6",
              "7",
              "8",
              "9",
              "10",
              "11",
          ],
          "B": ["a", "a", "a", "b", "b", "b", "c", "c", "c", "d", "d"],
          "C": in_vals,
      }
  
      df = DataFrame(source_dict)
      result = df.groupby("B").C.is_monotonic_decreasing
      index = Index(list("abcd"), name="B")
      expected = Series(index=index, data=out_vals, name="C")
      tm.assert_series_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, Index, Series
  import pandas._testing
label_desc: Use image node_id 2 for calling a global function with example usage: test_is_monotonic_decreasing(in_vals, out_vals) without return types