http://en.wikipedia.org/wiki/Butterworth_filter

http://ocw.mit.edu/resources/res-6-007-signals-and-systems-spring-2011/lecture-notes/MITRES_6_007S11_lec24.pdf

http://www.ece.uah.edu/courses/ee426/Butterworth.pdf

http://experimentalistsanonymous.com/diy/Schematics/Compressors%20Gates%20and%20Limiters/Butterworth%20filter.pdf

http://www.electronics-tutorials.ws/filter/filter_8.html

http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/bu.htm

http://en.wikipedia.org/wiki/File:Butterworth_filter_bode_plot.svg

http://www.ece.uic.edu/~jmorisak/blpf.html

http://de.wikipedia.org/wiki/Butterworth-Filter

http://www.calculatoredge.com/electronics/bw%20pi%20low%20pass.htm

http://www.kwon3d.com/theory/filtering/lpass.html#sys

http://www.google.com/url?sa=t&rct=j&q=butterworth+filter+coefficients++ppt&source=web&cd=5&cad=rja&ved=0CEQQFjAE&url=http%3A%2F%2Fwww.isip.piconepress.com%2Fcourses%2Ftemple%2Fece_3512%2Flectures%2Fcurrent%2Flecture_25.ppt&ei=6LcUUca_H4zesgaW5oD4Cw&usg=AFQjCNHcPJ2SBCn-hLi13Uxl9MO-7uW8_g&bvm=bv.42080656,d.Yms

http://www.google.com/url?sa=t&rct=j&q=butterworth+filter+coefficients++ppt&source=web&cd=7&cad=rja&ved=0CE4QFjAG&url=http%3A%2F%2Fwww.cmlab.csie.ntu.edu.tw%2FDSPCourse%2Fslide%2FLecture10.ppt&ei=6LcUUca_H4zesgaW5oD4Cw&usg=AFQjCNEO9J3YzEROsN3djamErNZtkssNqw&bvm=bv.42080656,d.Yms

http://ocw.mit.edu/resources/res-6-007-signals-and-systems-spring-2011/assignments/MITRES_6_007S11_hw24_sol.pdf

https://ccrma.stanford.edu/~jos/fp/Butterworth_Lowpass_Filter_Example.html

http://www.radio-electronics.com/info/rf-technology-design/rf-filters/butterworth-rf-filter-calculations-formulae-equations.php

http://home.sandiego.edu/~ekim/e194rfs01/ex5_3butter.pdf

http://eceweb1.rutgers.edu/~orfanidi/ece521/notes.pdf 

http://www.matheonics.com/Tutorials/Butterworth.html