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An Introduction to Combinatorial Analysis
By John Riordan PRINCETON UNIVERSITY PRESS
Copyright © 1978 Bell Telephone Laboratories, Inc.
All rights reserved.
ISBN: 978-0-691-08262-2
CHAPTER 1
Permutations and Combinations
1. INTRODUCTION
This chapter summarizes the simplest and most widely used material of the theory of combinations. Because it is so familiar, having been set forth for a generation in textbooks on elementary algebra, it is given here with a minimum of explanation and exemplification. The emphasis is on methods of reasoning which can be employed later and on the introduction of necessary concepts and working tools. Among the concepts is the generating function, the introduction of which leads to consideration of both permutations and combinations in great generality, a fact which seems insufficiently known.
Most of the proofs employ in one way or another either or both of the following rules.
Rule of Sum: If object A may be chosen in m ways, and B in n other ways, "either A or B" may be chosen in m + n ways.
Rule of Product: If object A may be chosen in m ways, and thereafter B in n ways, both "A and B" may be chosen in this order in mn ways.
These rules are in the nature of definitions (or tautologies) and need to be understood rather than proved. Notice that, in the first, the choices of A and B are mutually exclusive; that is, it is impossible to choose both (in the same way). The rule of product is used most often in cases where the order of choice is immaterial, that is, where the choices are independent, but the possibility of dependence should not be ignored.
The basic definitions of permutations and combinations are as follows:
Definition. An r-permutation of n things is an ordered selection or arrangement of r of them.
Definition. An r-combination of n things is a selection of r of them without regard to order.
A few points about these should be noted. First, in either case, nothing is said of the features of the n things; they may be all of one kind, some of one kind, others of other kinds, or all unlike. Though in the simpler parts of the theory, they are supposed all unlike, the general case is that of k kinds, with n, things of the jth kind and n = n1 + n2 + ··· + nk. The set of numbers (n1, n2, ···, nk) is called the specification of the things. Next, in the definition of permutations, the meaning of ordered is that two selections are regarded as different if the order of selection is different even when the same things are selected; the r-permutations may be regarded as made in two steps, first the selection of all possible sets of r (the r-combinations), then the ordering of each of these in all possible ways. For example, the 2-permutations of 3 distinct things labeled 1, 2, 3 are 12, 13, 23; 21, 31, 32; the first three of these are the 2-combinations of these things.
In the older literature, the r-permutations of n things are called variations, r at a time, for r< n, the term permutation being reserved for the ordering operation on all n things, as is natural in the theory of groups, as noticed in Chapter 4. This usage is followed here by taking the unqualified term, permutation, as always meaning n-permutation.
2. r-PERMUTATIONS
The rules and definition will now be applied to obtain the simplest and most useful enumerations.
2.1. Distinct Things
The first of the members of an r-permutation of n distinct things may be chosen in n ways, since the n are distinct. This done, the second may be chosen in n - 1 ways, and so on until the rth is chosen in n - r + 1 ways. By repeated application of the rule of product, the number required is
P(n, r) = n(n - 1) ··· (n - r + 1), n ≥ r (1)
If r = n, this becomes
P(n, n) = n(n - 1) ··· 1 = n! (2)
that is, the product of all integers from 1 to n, which is called n-factorial, is written n! as above. P(n, 0), which has no combinatorial meaning, is taken by convention as unity.
Using (2), (1) may be rewritten
P(n, r) = n!/(n - r)! = P(n, n)/P(n - r, n - r)
and is also given the abbreviation
P(n, r) = (n)r
The last is called the falling r-factorial of n; since the same notation is also used in the theory of the hypergeometric function to indicate n(n + 1) ··· (n + r - 1), the word "falling" in the statement is necessary to avoid ambiguity.
The relation P(n, n) = P(n, r)P(n - r, n - r) appearing above also follows from the rule of product.
It is also interesting to note the following recurrence relation
P(n, r) = P(n- 1, r) + rP(n - 1, r - 1) = (n - 1)r + r(n - 1)r-1 (3)
This follows by simple algebra from (1), writing n = n - r + r; it also follows by classifying the r-permutations as to whether they do not or do contain a given thing. If they do not, the number is P(n - 1, r); if they do, there are r positions in which the given thing may appear and P(n - 1, r - 1) permutations of the n - 1 other things.
Example 1. The 12 2-permutations of 4 distinct objects (n = 4, n = 2) labeled 1,2, 3, and 4 are:
12, 13, 14, 23, 24, 34 21, 31, 41, 32, 42, 43
2.2. The Number of Permutations of n Things, p of Which Are of One Kind, q of Another, etc.
Here, permutations n at a time only are considered; that is, the term "permutation" is used in the unqualified sense. Let x be the number required. Suppose the p like things are replaced by p new things, distinct from each other and from all other kinds of things being permuted. These may be permuted in p! ways (by equation (2)); hence the number of permutations of the new set of objects is xp!. The same goes for every other set of like things and, since finally all things become unlike and the number of permutations of n unlike things is n!,
xp! q! ··· = n!
or
x = n!/p!q! ···, p + q + ··· = n (4)
The right of (4) is a multinomial coefficient, that is, a coefficient, in the expansion of (a + b + ···)n. It is also the number of arrangements of n unlike things into unlike cells or boxes, p in the first, q in the second, and so on, without regard to order in any cell, as will be shown in Chapter 5.
Example 2. The number of arrangements on one shelf of n different books, for each of which there are m copies, is (nm)!/(m!)n.
The more general problem of finding the number of r-permutations of things of general specification (p of one kind, q of another, etc.) will be treated in a later section of this chapter by the method of generating functions.
2.3. r-Permutations with Unrestricted Repetition
Here there are n kinds of things, and an unlimited number of each kind or, what is the same thing, any object after being chosen is replaced in the stock. Hence, each place in an r-permutation may be filled in n different ways, because the stock of things is unaltered by any choice. By the rule of product, the number of permutations in question, that is, the r-permutations with repetition of n things, is,
U(n, r) = nr (5)
Example 3. The number of ways an r-hole tape (as used in teletype and computing machinery) can be punched is 2r. Here there are two "objects", clear (unpunched) and punched tape, for each of the r positions of any line on the tape.
In the language of statistics, U(n, r) is for sampling with replacement, as contrasted with P(n, r) which is for sampling without replacement.
3. COMBINATIONS
3.1. r-Combinations of n Distinct Things
The simplest derivation is to notice that each combination of r distinct things may be ordered in r! ways, and so ordered is an r-permutation; hence, if C(n, r) is the number in question
r! C(n, r) = P(n, r) = n(n - 1) ··· (n - r + 1), n ≥ r
and
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
where the last symbol is that usual for binomial coefficients, that is, the coefficients in the expansion of (a + b)n. (Cnr, Crn, nCr, and (n, r) are alternative notations; the first two are difficult in print and the superscripts are liable to confusion with exponents, the first subscript of the third is sometimes hard to associate with the C which it modifies, the C is strictly unnecessary since the numbers are not necessarily associated with combinations, but all have the sanction of some usage in and out of print.)
C(n, 0), the number of combinations of n things zero at a time, has no combinatorial meaning, but is given by (6) to be unity, which is the usual convention. The values
C(n, r) = 0, r < 0 and r > n
are also in agreement with (6). C(-n, r) has no combinatorial meaning, but it may be noticed that, from (6)
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and
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
Example 4. The 6 combinations of 4 distinct objects taken two at a time (n = 4, r = 2), labeling the objects 1, 2, 3, and 4, are:
12, 13, 14, 23, 24, 34
Another derivation of (6) is by recurrence and the rule of sum. The combinations may be divided into those which include a given thing, say the first, and those which do not. The number of those of the first kind is C(n - 1, r - 1), since fixing one element of a combination reduces both n and r by one; the number of those of the second kind, for a similar reason, is C(n - 1, r). Hence,
C(n, r) = C(n - 1, r - 1) + C(n - 1, r), n ≥ r (7)
Using mathematical induction, it may be shown that (6) is the only solution of (7) which satisfies the boundary conditions C(n, 0) = C(1, 1) = 1, C(1, 2) = C(1, 3) = ··· = 0.
Equation (7) is an important formula because it is the basic recurrence for binomial coefficients. Notice that by iteration it leads to
C(n, r) = C(n - 1, r - 1) + C(n - 2, r - 1) + ··· + C( r - 1, r - 1) (8) = C(n - 1, r) + C(n - 2, r - 1) + ··· + C(n - 1 - r, 0)
The first of these classifies the r-combinations of n numbered things according to the size of the smallest element, that is, C(n - k, r - 1) is the number of r-combinations in which the smallest element is k. The similar combinatorial proof of the second does not lend itself to simple statement.
For numerical concreteness, a short table of the numbers C(n, r), a version of the Pascal triangle, is given below. Note how easily one row may be filled in from the next above by (7). Note also the symmetry, that is C(n, r) = C(n, n - r); this follows at once from (6), and is also evident from the fact that a selection of r determines the n-r elements not selected.
3.2. Combinations with Repetition
The number sought is that of r-combinations of n distinct things, each of which may appear indefinitely often, that is, 0 to r times. This is a function of n and r, say f(n, r).
The most natural method here seems to be that of recurrence and the rule of sum. Suppose the things are numbered 1 to n; then the combinations either contain 1 or they do not. If they do, they may contain it once, twice, and so on, up to r times, but, in any event, if one of the appearances of element 1 is crossed out, f(n, r - 1) possible combinations are left. If they do not, there are f(n - 1, r) possible combinations. Hence,
f(n, r) = f(n, r - 1) + f(n - 1, r) (9)
If r = 1, no repetition of elements is possible and f(n, 1) = n. (Note that this determines f(n, 0) = f(n, 1) - f(n - 1, 1) as 1, a natural convention.) If n = 1, only one combination is possible whatever r, so f(1, r) = 1. Now
f(n, 2) = f(n, 1) + f(n - 1, 2) = f(n, 1) + f(n - 1, 1) + f(n - 2, 2)
and if this is repeated until the appearance of f(1, 2), which is unity,
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
by the formula for the sum of an arithmetical series, or by the first of equations (8).
Similarly
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
again by the first of equations (8).
The form of the general answer, that is, the number of r-combinations with repetition of n distinct things, now is clearly
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)
and it may be verified without difficulty that this satisfies (9) and its boundary conditions: f(n, 1) = n, f(1, r) = 1.
Example 5. The number of combinations with repetition, 2 at a time, of 4 things numbered 1 to 4, by equation (10), is 10; divided into two parts as in equation (9), these combinations are
11, 12, 13, 14 22, 23, 24, 33, 34, 44
The result in (10) is so simple as to invite proofs of equal simplicity. The best of these, which may go back to Euler, is as follows. Consider any one of the r-combinations with repetition of n numbered things, say, c1c2 ··· cr in rising order (with like elements taken to be rising), where, of course, because of unlimited repetition, any number of consecutive c's may be alike. From this, form a set d1d2 ··· dr by the rules: d1 - c1 + 0, d2 = c2 + 1, d1 = ci + i - 1, ···, dr - cr + r - 1; hence, whatever the c's, the d's are unlike. It is clear that the sets of c's and d's are equinumerous; that is, each distinct r-combination produces a distinct set of d's, and vice versa. The number of sets of d's is the number of r-combinations (without repetition) of elements numbered 1 to n + r - 1, which is C(n + r - 1, r), in agreement with (10); for example, the d's corresponding to the r-combinations of Example 5 are (in the same order)
12, 13, 14, 15 23, 24, 25, 34, 35, 45
4. GENERATING FUNCTION FOR COMBINATIONS
The enumerations given above may be unified and generalized by a relatively simple mathematical device, the generating function.
For illustration, consider three objects labeled x1, x2, and x3. Form the algebraic product
(1 + x1t)(1 + x2t)(1 + x3t)
Multiplied out and arranged in powers of t, this is
1 + (x1 + x2 + x3)t + (x1x2 + x1x3 + x2x3)t2 + x1x2x3t3
or, in the notation of symmetric functions,
1 + a1t + a2t2 + a3t3
where a1, a2, and a3 are the elementary symmetric functions of the three variables x1, x2, and x3. These symmetric functions are identified by the equation ahead, and it will be noticed that ar, r = 1, 2, 3, contains one term for each combination of the three things taken r at a time. Hence, the number of such combinations is obtained by setting each xl, to unity, that is,
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
(Continues...)
Excerpted from An Introduction to Combinatorial Analysis by John Riordan. Copyright © 1978 Bell Telephone Laboratories, Inc.. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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