http://eprints.umm.ac.id/47718/3/BAB%202.pdf
http://eprints.umpo.ac.id/5033/3/BAB%202.pdf
http://repository.ump.ac.id/2264/3/ADE%20WEGI%20PAMBUDI%20BAB%20II.pdf
https://repository.uhn.ac.id/bitstream/handle/123456789/1403/Kaisar%20E.S.%20Sinurat.pdf?sequence=1&isAllowed=y
https://www.ncbi.nlm.nih.gov/books/NBK513332/