code: def autocorr(s, axis=-1):
      """Returns the autocorrelation of signal s at all lags. Adheres to the
      definition r(k) = E{s(n)s*(n-k)} where E{} is the expectation operator.
      """
      N = s.shape[axis]
      S = np.fft.fft(s, n=2 * N - 1, axis=axis)
      sxx = np.fft.ifft(S * S.conjugate(), axis=axis).real[:N]
      return sxx / N
  
imports: import numpy
  from numpy.testing import assert_array_almost_equal
  import matplotlib.pyplot
  from statsmodels import regression
  from statsmodels.tsa.arima_process import arma_generate_sample, arma_impulse_response
  from statsmodels.tsa.arima_process import arma_acovf, arma_acf
  from statsmodels.tsa.arima.model import ARIMA
  from statsmodels.tsa.stattools import acf, acovf
  from statsmodels.graphics.tsaplots import plot_acf
label_desc: Use image node_id 11 for calling a global function with example usage: autocorr(s, axis) and returns: unknown