code: def var(n):
      return np.sqrt((n**2 - 1) / 12)
  
file_path: pyts/pyts/classification/tests/test_tsbf.py
imports: import numpy
  import pytest
  import re
  from pyts.classification import TSBF
  from pyts.classification.tsbf import IntervalFeatureExtractor
  from pyts.classification.tsbf import extract_features, histogram
  from sklearn.tree import DecisionTreeClassifier
label_desc: Use image node_id 1 for calling a global function with example usage: var(n) and returns: np