There are many branches of mathematics, but within statistics the most important part is probability theory, which enables us to reason explicitly about probabilities.
Also used in hcistats2e: Chap. 1: pages 1, 3; Chap. 2: page 19; Chap. 4: page 56; Chap. 6: pages 66, 71; Chap. 7: page 86; Chap. 10: page 116; Chap. 13: page 156
Also known as: mathematics of probability, maths