code: def test_expm():
      """Test matrix exponential"""
      C = 2 * np.eye(n_channels)
      Ctrue = np.exp(2) * np.eye(n_channels)
      assert_array_almost_equal(expm(C), Ctrue, decimal=10)
  
file_path: pyRiemann/tests/test_utils_base.py
imports: import pytest
  import numpy
  from numpy.testing import assert_array_almost_equal
  from pyriemann.utils.base import expm, invsqrtm, logm, powm, sqrtm, nearest_sym_pos_def
  from pyriemann.utils.mean import mean_riemann
  from pyriemann.utils.test import is_pos_def, is_sym_pos_def
label_desc: Use image node_id 1 for calling a global function with example usage: test_expm() without return types