code: def test_sofm_decay_function(self):
      actual_output = sofm.decay_function(12, 10, reduction_rate=10)
      self.assertEqual(6, actual_output)
  
      actual_output = sofm.decay_function(12, 20, reduction_rate=10)
      self.assertEqual(4, actual_output)
  
      actual_output = sofm.decay_function(12, 30, reduction_rate=10)
      self.assertEqual(3, actual_output)
  
imports: import math
  import numpy
  from neupy import algorithms
  from neupy.exceptions import WeightInitializationError
  from neupy.algorithms.competitive import sofm
  from neupy.algorithms.competitive.neighbours import gaussian_df
  from base import BaseTestCase
label_desc: Use image node_id 3 for calling the SOFMUtilsFunctionTestsCase obj's underlying member method code with example usage: obj.test_sofm_decay_function() without return types