University Mathematics Handbook (2015)
XV. Combinatorics and Newton's Binomial
Chapter 3. Combination
a. The combination of 
out of 
distinct objects is a choice of out of given distinct objects, when the order of choice is insignificant and no object can be chosen more than one time.
b. One combination of out of distinct objects is different from another combination of out of same objects in at least one term.
c. The number of combinations is denoted as 
.
d. 