Finite Impulse Response for butterworth filter
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-341-discrete-time-signal-processing-fall-2005/lecture-notes/lec08.pdf
Discrete-Time Signal Processing (Second Edition)
Alan V. Oppenheim, Ronald W. Schafer with John R. Book
http://www.ingelec.uns.edu.ar/pds2803/Materiales/LibrosPDF/Oppenheim/TOC.htm