code: def _1d_test_function(x):
      # Test function used in Wahba's "Spline Models for Observational Data".
      # domain ~= (0, 3), range ~= (-1.0, 0.2)
      x = x[:, 0]
      y = 4.26 * (np.exp(-x) - 4 * np.exp(-2 * x) + 3 * np.exp(-3 * x))
      return y
  
imports: import pickle
  import pytest
  import numpy
  from numpy.linalg import LinAlgError
  from numpy.testing import assert_allclose, assert_array_equal
  from scipy.stats.qmc import Halton
  from scipy.spatial import cKDTree
  from scipy.interpolate._rbfinterp import _AVAILABLE, _SCALE_INVARIANT, _NAME_TO_MIN_DEGREE, _monomial_powers, RBFInterpolator
  from scipy.interpolate import _rbfinterp_pythran
label_desc: Use image node_id 2 for calling a global function with example usage: _1d_test_function(x) and returns: y