code: def _coeff4(N, a0, a1, a2, a3):
      
      if N == 1:
          return ones(1)
  
      n = arange(0, N)
      N1 = N - 1.
  
      w = a0 -a1*cos(2.*pi*n / N1) + a2*cos(4.*pi*n / N1) - a3*cos(6.*pi*n / N1)
  
      return w
docs: a common internal function to some window functions with 4 coeffs
  
  
      For the blackmna harris for instance, the results are identical to octave if N is odd
      but not for even values...if n =0 whatever N is, the w(0) must be equal to a0-a1+a2-a3, which
      is the case here, but not in octave...