permutations and combinations

Number of ways a subset of objects can be selected from a given set of objects.

In a permutation, order is important; in a combination, it is not. Thus, there are six permutations of the letters A, B, C selected two at a time (AB, AC, BC, BA, CA, CB) yet only three combinations (AB, AC, BC). The number of permutations of r objects chosen from a set of n objects, expressed in factorial notation, is n! ÷ (n -r)! The number of combinations is n! ÷ [r!(n -r)!]. The (r + 1)st coefficient in the binomial expansion of (x + y)ⁿ coincides with the combination of n objects chosen r at a time (see binomial theorem). Probability theory evolved from the study of gambling, including figuring out combinations of playing cards or permutations of win-place-show possibilities in a horse race, and such counting methods played an important role in its development in the 17th century.

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▪ mathematics

      the various ways in which objects from a set may be selected, generally without replacement, to form subsets. This selection of subsets is called a permutation when the order of selection is a factor, a combination when order is not a factor. By considering the ratio of the number of desired subsets to the number of all possible subsets for many games of chance in the 17th century, the French mathematicians Blaise Pascal (Pascal, Blaise) and Pierre de Fermat (Fermat, Pierre de) gave impetus to the development of combinatorics and probability theory.

      The concepts of and differences between permutations and combinations can be illustrated by examination of all the different ways in which a pair of objects can be selected from five distinguishable objects—such as the letters A, B, C, D, and E. If both the letters selected and the order of selection are considered, then the following 20 outcomes are possible:

      Each of these 20 different possible selections is called a permutation. In particular, they are called the permutations of five objects taken two at a time, and the number of such permutations possible is denoted by the symbol ₅P₂, read “5 permute 2.” In general, if there are n objects available from which to select, and permutations (P) are to be formed using k of the objects at a time, the number of different permutations possible is denoted by the symbol _nP_k. A formula for its evaluation is

_nP_k = n!/(n − k)!

The expression n!—read “n factorial”—indicates that all the consecutive positive integers from 1 up to and including n are to be multiplied together, and 0! is defined to equal 1. For example, using this formula, the number of permutations of five objects taken two at a time is

      (For k = n, _nP_k = n! Thus, for 5 objects there are 5! = 120 arrangements.)

      For combinations, k objects are selected from a set of n objects to produce subsets without ordering. Contrasting the previous permutation example with the corresponding combination, the AB and BA subsets are no longer distinct selections; by eliminating such cases there remain only 10 different possible subsets—AB, AC, AD, AE, BC, BD, BE, CD, CE, and DE.

      The number of such subsets is denoted by _nC_k, read “n choose k.” For combinations, since k objects have k! arrangements, there are k! indistinguishable permutations for each choice of k objects; hence dividing the permutation formula by k! yields the following combination formula:

      This is the same as the (n, k) binomial coefficient (see binomial theorem). For example, the number of combinations of five objects taken two at a time is

      The formulas for _nP_k and _nC_k are called counting formulas since they can be used to count the number of possible permutations or combinations in a given situation without having to list them all.

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