http://www.eng.auburn.edu/~strouce/class/elec2200/elec2200-8.pdf
http://www.ee.surrey.ac.uk/Projects/CAL/digital-logic/combindex.html
http://web.ee.nchu.edu.tw/~cpfan/FY92b-digital/Chapter4.pdf
http://www2.nau.edu/~sh295/EE110/PracticeProbs208sols.pdf
http://www.dauniv.ac.in/downloads/Digitalsystems_PPTs/DigDesignCh09L1.pdf
http://web.ist.utl.pt/luis.tarrataca/classes/computer_architecture/Chapter11-DigitalLogic.pdf
https://ece.uwaterloo.ca/~msachdev/ECE223/comb.pdf
http://www.eng.ucy.ac.cy/ece210/lecture_notes/lcdf3_chap_04.pdf
http://www.srmuniv.ac.in/ramapuram/sites/ramapuram/files/part1.pdf