code: def _1d_test_function(x, xp):
      # 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 * (xp.exp(-x) - 4 * xp.exp(-2 * x) + 3 * xp.exp(-3 * x))
      return y
  
imports: import pickle
  import pytest
  import warnings
  import numpy
  from numpy.linalg import LinAlgError
  import cupy
  import cupyx
  from cupy import testing
  import cupyx.scipy.interpolate
  from cupyx.scipy.interpolate._rbfinterp import _AVAILABLE, _SCALE_INVARIANT, _NAME_TO_MIN_DEGREE, NAME_TO_FUNC, _monomial_powers, polynomial_matrix, kernel_matrix
label_desc: Use image node_id 4 for calling a global function with example usage: _1d_test_function(x, xp) and returns: y