Return the Hanning window.
The Hanning window is a taper formed by using a weighted cosine.
Parameters:  M : int


Returns:  out : ndarray, shape(M,)

Notes
The Hanning window is defined as
The Hanning was named for Julius van Hann, an Austrian meterologist. It is also known as the Cosine Bell. Some authors prefer that it be called a Hann window, to help avoid confusion with the very similar Hamming window.
Most references to the Hanning window come from the signal processing literature, where it is used as one of many windowing functions for smoothing values. It is also known as an apodization (which means “removing the foot”, i.e. smoothing discontinuities at the beginning and end of the sampled signal) or tapering function.
References
[34]  Blackman, R.B. and Tukey, J.W., (1958) The measurement of power spectra, Dover Publications, New York. 
[35]  E.R. Kanasewich, “Time Sequence Analysis in Geophysics”, The University of Alberta Press, 1975, pp. 106108. 
[36]  Wikipedia, “Window function”, http://en.wikipedia.org/wiki/Window_function 
[37]  W.H. Press, B.P. Flannery, S.A. Teukolsky, and W.T. Vetterling, “Numerical Recipes”, Cambridge University Press, 1986, page 425. 
Examples
>>> from numpy import hanning
>>> hanning(12)
array([ 0. , 0.07937323, 0.29229249, 0.57115742, 0.82743037,
0.97974649, 0.97974649, 0.82743037, 0.57115742, 0.29229249,
0.07937323, 0. ])
Plot the window and its frequency response:
>>> from numpy.fft import fft, fftshift
>>> import matplotlib.pyplot as plt
>>> window = np.hanning(51)
>>> plt.subplot(121)
>>> plt.plot(window)
>>> plt.title("Hann window")
>>> plt.ylabel("Amplitude")
>>> plt.xlabel("Sample")
>>> A = fft(window, 2048) / 25.5
>>> mag = abs(fftshift(A))
>>> freq = np.linspace(0.5,0.5,len(A))
>>> response = 20*np.log10(mag)
>>> response = np.clip(response,100,100)
>>> plt.subplot(122)
>>> plt.plot(freq, response)
>>> plt.title("Frequency response of the Hann window")
>>> plt.ylabel("Magnitude [dB]")
>>> plt.xlabel("Normalized frequency [cycles per sample]")
>>> plt.axis('tight'); plt.show()