http://www.math.scu.edu.tw/teacher/Chieping/Lebesgue-measure.html#top
https://www.csie.ntu.edu.tw/~b89089/book/Royden/Royden_3.pdf
https://en.wikipedia.org/wiki/Lebesgue_measure
http://mathworld.wolfram.com/LebesgueMeasure.html
http://faculty.bard.edu/belk/math461/LebesgueMeasure.pdf
https://www.math.ucdavis.edu/~hunter/measure_theory/measure_notes_ch2.pdf