Combination Locks and Permutations

Transcription

1 Combination Locks and Permutations An Exploration Through Analysis Tim Sasaki Western Oregon University April 9, 2011 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

2 Introduction Suppose we have a combination lock with buttons numbered 1 through n. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

3 Introduction Suppose we have a combination lock with buttons numbered 1 through n. Combinations are entered by pressing buttons in sequence, with each button being pushed only once. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

4 Introduction Suppose we have a combination lock with buttons numbered 1 through n. Combinations are entered by pressing buttons in sequence, with each button being pushed only once. How many combinations are possible? Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

5 Introduction Suppose we have a combination lock with buttons numbered 1 through n. Combinations are entered by pressing buttons in sequence, with each button being pushed only once. How many combinations are possible? If we push buttons one at a time, then the number of combinations is simply n! since we are not repeating buttons. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

6 Introduction Suppose we have a combination lock with buttons numbered 1 through n. Combinations are entered by pressing buttons in sequence, with each button being pushed only once. How many combinations are possible? If we push buttons one at a time, then the number of combinations is simply n! since we are not repeating buttons. However, what if we are allowed to push multiple buttons at once? Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

7 Introduction An example for n = 4 Possible combinations are: {1,2},{3},{4} {2,3,4},{1} {1,3},{2,4} {1},{2},{3},{4} In particular, the set of multiple-button-push combinations contains the set of single-button-push combinations, so we know there are at least n! combinations (the only instance when the number of combinations is actually equal to n! is for n {0,1}). Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

8 Introduction Exhaustive checking works fine for small n. Combinations for n = 3 ({1}, {2}, {3}), ({1}, {3}, {2}), ({2}, {1}, {3}), ({2}, {3}, {1}), ({3}, {1}, {2}), ({3}, {2}, {1}), ({1, 2}, {3}), ({1, 3}, {2}), ({2, 3}, {1}), ({1}, {2, 3}),({2}, {1, 3}), ({3}, {1, 2}), ({1, 2, 3}) There are 13 possible combinations for n = 3. However, the number of combinations as n increases becomes very large. We know that there are at least n! combinations, and the factorials blow up quickly. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

9 Introduction For n = 6, this gives us at least 6! = 720 combinations. Then, we are motivated to try and derive a formula to compute the number of combinations for a lock with n buttons, since writing out each of 720 combinations is not desirable. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

10 Introduction Let a n be the number of combinations for an n-button lock. Then, note that a 0, the number of combinations for a 0-button lock, is equal to 1 (the only combination is no buttons pressed). Further, for any n > 0, a valid combination comprises an initial push of k buttons, for 1 k n, followed by a combination formed with the remaining n k buttons. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

11 Introduction This leads us to a recurrence relation: a n = ( ) ( ) ( ) ( ) ( ) n n n n n n a n k = a n 1 + a n a 1 + a 0, k 1 2 n 1 n k=1 where ( n k) represents the number of ways to pick the initial k buttons and a n k represents the number of ways to choose all of the remaining n k buttons. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

12 Introduction Let s try out our formula using n = 3. Using our recurrence relation, ( ) ( ) ( ) a 3 = a 2 + a 1 + a ( ) ( ) ( ) = (3) + (1) + (1) = (3 3) + (3 1) + (1 1) = 13 This matches the value we had earlier. Naturally, we would like to try and find a closed-form equation instead of a recursive one. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

13 Introduction Let s try out our formula using n = 3. Using our recurrence relation, ( ) ( ) ( ) a 3 = a 2 + a 1 + a ( ) ( ) ( ) = (3) + (1) + (1) = (3 3) + (3 1) + (1 1) = 13 This matches the value we had earlier. Naturally, we would like to try and find a closed-form equation instead of a recursive one. This is a task we will accomplish through the use of some infinite series and integrals. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

14 Introduction Let s try out our formula using n = 3. Using our recurrence relation, ( ) ( ) ( ) a 3 = a 2 + a 1 + a ( ) ( ) ( ) = (3) + (1) + (1) = (3 3) + (3 1) + (1 1) = 13 This matches the value we had earlier. Naturally, we would like to try and find a closed-form equation instead of a recursive one. This is a task we will accomplish through the use of some infinite series and integrals. For the duration of the talk, we will adopt the convention that 0 0 = 1 strictly for notational convenience. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

15 A new recursive formula We now take our relation ( ) ( ) ( ) ( ) ( ) n n n n n n a n = a n k = a n 1 + a n a 1 + a 0, k 1 2 n 1 n k=1 and put in the values of the binomial coefficients: n! a n = (n 1)!1! a n 1 + ( = n! n! (n 2)!2! a n a n 1 (n 1)!1! + a n 2 (n 2)!2! + + a 1 1!(n 1)! + a 0 0!n! n! 1!(n 1)! a 1 + n! 0!n! a 0 ). Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

16 A new recursive formula We now take our relation ( ) ( ) ( ) ( ) ( ) n n n n n n a n = a n k = a n 1 + a n a 1 + a 0, k 1 2 n 1 n k=1 and put in the values of the binomial coefficients: n! a n = (n 1)!1! a n 1 + ( = n! n! (n 2)!2! a n a n 1 (n 1)!1! + a n 2 (n 2)!2! + + a 1 1!(n 1)! + a 0 0!n! n! 1!(n 1)! a 1 + n! 0!n! a 0 ). We notice a relationship in the terms of the sum between each a k and the first factorial in the denominator. We thus define a new term b n = a n / n! and rewrite the relation: a n = n! ( bn 1 1! + b n 2 2! + + b ) 0 n! Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

17 A new recursive formula ( bn 1 a n = n! + b n b ) 0 1! 2! n! Finally, since b n = a n / n!, we divide by n! to get b n = a ( n n! = bn 1 + b n b ) 0 = 1! 2! n! n k=1 b n k. k! This relation is a bit simpler than our original one, so it may be easier to find a closed equation for b n first and then get one for a n. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

18 The calculus begins Theorem 1 For all n > 0, 1 2(ln2) n b n 1 (ln2) n We consider the generating function f (x) = b n x n. n=0 It can be shown using Theorem 1 that the series is absolutely convergent for x < ln2. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

19 And continues. Then, for x < ln 2, f (x) = b 0 + = 1 + b n x n n=1 ( b0 1! x1 ( b0 ) ( b1 + 1! x2 + b ) 0 2! x2 = 1 + 1! x1 + b 1 1! x2 + b 2 1! x3 + b n k = 1 + x n k=1 n=k k! x k = 1 + b n x n k! k=1 n=0 = 1 + (e x 1)f (x) ( b2 + 1! x3 + b 1 2! x3 + b 0 3! x3 ( b0 2! x2 + b 1 2! x3 + ) + From this, we get that f (x) = 1 2 e for x < ln2. x ) + ) + Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

20 And continues.. If b n x n is the power series representation of f at 0, then we know that n=0 the coefficients, b n, have the form b n = f (n) (0). n! Recall that b n = a n / n!, which implies that or a n = f (n) (0), a n = d n ( 1 dx n 2 e x ). x=0 However, this formula is still problematic, since we must calculate each intermediary derivative to get to the n th one if we wish to use it. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

21 And continues... To get past this problem, we can view f (x) as a geometric series: Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

22 And continues... To get past this problem, we can view f (x) as a geometric series: e x = 2 (1) 1 2 (2 ex ) = 1/2 1 e x /2 = 1 ( e x ) k, 2 k=0 2 as long as the series converges, which is for e x /2 < 1. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

23 And continues... To get past this problem, we can view f (x) as a geometric series: e x = 2 (1) 1 2 (2 ex ) = 1/2 1 e x /2 = 1 ( e x ) k, 2 k=0 2 as long as the series converges, which is for e x /2 < 1. Since e x /2 is always positive, this becomes e x 2 < 1 = ex < 2 = x < ln2. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

24 And continues... To get past this problem, we can view f (x) as a geometric series: e x = 2 (1) 1 2 (2 ex ) = 1/2 1 e x /2 = 1 ( e x ) k, 2 k=0 2 as long as the series converges, which is for e x /2 < 1. Since e x /2 is always positive, this becomes e x Then, because 0 < ln 2, 2 < 1 = ex < 2 = x < ln2. [ a n = d n ( 1 e x ) ] k dx n = 1 ( e k n 0 ) k = 1 k n x=0 2 k. k=0 k=0 k=0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

25 And now, integrals We can approximate the sum k=0 k n 2 k 0 x n 2 x dx, Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

26 And now, integrals We can approximate the sum k=0 k n 2 k 0 x n 2 x dx, but first we want to make the substitutions u = x ln2 = x = u/ln2 and dx = 1/ln2 du: Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

27 And now, integrals We can approximate the sum k=0 k n 2 k 0 x n 2 x dx, but first we want to make the substitutions u = x ln2 = x = u/ln2 and dx = 1/ln2 du: x n 0 2 x dx = 0 1 = (ln2) n+1 (u/(ln2)) n 2 u/(ln2) u n 0 1 ln2 du = 1 (ln2) n+1 0 e u du. u n ( 2 (1/ln2) ) u du Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

28 Integrals, continued We focus now on the new integral and use integration by parts n times (choosing the constituent parts wisely): u n 0 du = lim eu b n k=0 n! (n k)! u n k e u b 0 = n n! (n k)! k=0 0 n k e 0. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

29 Integrals, continued We focus now on the new integral and use integration by parts n times (choosing the constituent parts wisely): u n 0 du = lim eu b n k=0 n! (n k)! u n k e u b 0 = n n! (n k)! k=0 0 n k e 0. We note that every term in the sum is equal to 0 except the last, which is n! 0 0 (n n)! e 0 = n! 1 0! 1 = n!. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

30 Integrals, continued We focus now on the new integral and use integration by parts n times (choosing the constituent parts wisely): u n 0 du = lim eu b n k=0 n! (n k)! u n k e u b 0 = n n! (n k)! k=0 0 n k e 0. We note that every term in the sum is equal to 0 except the last, which is n! 0 0 (n n)! e 0 = n! 1 0! 1 = n!. Referring back to our estimation of a n, this gives us a n = 1 2 k=0 k n 2 k x n 2 x dx = 1 u n 2(ln2) n+1 0 e u du = n! 2(ln2) n+1. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

31 A brief recap We ve derived the following formulas for a n so far: 1 a n = ( ) ( ) ( ) ( ) ( ) n n n n n n a n k = a n 1 + a n a 1 + k 1 2 n 1 n k=1 2 a n = d n 3 a n = a n dx n ( 1 2 e x k n k=0 2 k n! 2(ln2) n+1 ) x=0 a 0 We now wish to determine just how accurate our last approximation is. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

32 Some more calculus We begin by defining a function g such that for n N, for all x [0, ), g (x) := x n /e x. The First Derivative Test will show us that g is increasing on [0, n/ ln 2] and decreasing on [n/ ln 2, ). Hence, g has a local maximum at x = n/ln2. We are going to use rectangles to approximate the integral 0 x n e x dx. Instead of using the height at left-or-right endpoints, however, we will be using the suprema (least upper bound) and infima (greatest lower bound) of g in each subinterval. These are called Darboux sums. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

33 A pictoral interlude Break the interval [0, ) into subintervals [0,1],[1,2],, and define j = n/ln2. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

34 Back to work Recall that g has a max at x = n/ln2. The value of g at this point is (n/ln2) n n n ( 2 n/ln2 = ( ) 2 1/ln2 n = n ) n. (ln2) n e ln2 Then, the sum of the areas of the rectangles from the previous slide is Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

35 Back to work Recall that g has a max at x = n/ln2. The value of g at this point is (n/ln2) n n n ( 2 n/ln2 = ( ) 2 1/ln2 n = n ) n. (ln2) n e ln2 Then, the sum of the areas of the rectangles from the previous slide is j k n ( 2 k + n ) n (k) n + e ln2 2 k k=1 k=j +1 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

36 Back to work Recall that g has a max at x = n/ln2. The value of g at this point is (n/ln2) n n n ( 2 n/ln2 = ( ) 2 1/ln2 n = n ) n. (ln2) n e ln2 Then, the sum of the areas of the rectangles from the previous slide is j k n ( k=1 2 k + n ) n (k) n + e ln2 k=j +1 2 k k n ( = 2 k + n ) n ( n ) n. = 2an + e ln2 e ln2 k=0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

37 An answer at last The approximation is clearly greater than the value of the integral, so this gives us x n 0 ( 2 x dx < 2a n ) n 1 ( n ) n n + = < an e ln2 2 e ln2 A similar process for an underestimation of the integral yields Hence, we have n! a n 2(ln2) n+1 < 1 ( n ) n. 2 e ln2 a n n! 2(ln2) n+1 < 1 ( n ) n, 2 e ln2 which means our approximation is within 1 2 ( n e ln2) n of an. n! 2(ln2) n+1. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

38 An unexpected result The error bound we found is a function that increases with n. Despite this, we can show that a n lim n n! /[ 2(ln2) n+1] = 1. The proof uses our derived error bounds, Stirling s Formula, and the Squeeze Theorem to show that lim n a n n! /[ 2(ln2) n+1] 1 = 0. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

39 An unexpected result This may initially seem counter-intuitive, since the error is increasing, but consider as a simple example n 3 + n 2 and n 3. The error between them is n 2, which increases quadratically with n, but n 3 + n 2 lim n n 3 = 1. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

40 Finally, some combinatorics Instead of approximating a n via the methods we have gone through, we can also simply just evaluate the infinite sum. For n 0, define Recalling our formula it is clear that h n (x) = a n = 1 2 k n x k. k=0 k=0 a n = 1 2 h n k n 2 k, So, by finding formulas for each h n, we can find a formula for a n. ( 1 2 ). Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

41 But first, some preliminary calculus (Ha!) Looking at h 0 (x), we have h 0 (x) = k 0 x k = k=0 k=0 x k = 1 1 x 1 < x < 1. We now look at the differentiation of the series: h n (x) = k n k x k 1 = k n+1 x k 1, k=0 k=0 from which we can get xh n (x) = k n+1 x k = h n+1 (x). k=0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

42 Yet another recurrence Given our newly found recurrence, xh n (x) = h n+1(x), we look at its particular values for n 5 : h 0 (x) = 1 1 x x h 1 (x) = (1 x) 2 h 2 (x) = x + x2 (1 x) 3 h 3 (x) = x + 4x2 + x 3 (1 x) 4 h 4 (x) = x + 11x2 + 11x 3 + x 4 (1 x) 5 h 5 (x) = x + 26x2 + 66x x 4 + x 5 (1 x) 6 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

43 Some patterns of note We notice some patterns in each h n : Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

44 Some patterns of note We notice some patterns in each h n : 1 The numerator contains a polynomial of degree n. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

45 Some patterns of note We notice some patterns in each h n : 1 The numerator contains a polynomial of degree n. 2 The coefficients of the polynomial appear to be symmetric. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

46 Some patterns of note We notice some patterns in each h n : 1 The numerator contains a polynomial of degree n. 2 The coefficients of the polynomial appear to be symmetric. 3 The denominator is always (1 x) n+1. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

47 Some patterns of note We notice some patterns in each h n : 1 The numerator contains a polynomial of degree n. 2 The coefficients of the polynomial appear to be symmetric. 3 The denominator is always (1 x) n+1. We define now A n,k to be the coefficient of x k in h n. For example, given A 4,3 = 11. h 4 (x) = x + 11x2 + 11x 3 + x 4 (1 x) 5, Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

48 This is the last recurrence relation Theorem 2 For all n 1, n k=1 h n (x) = A n,k x k (1 x) n+1 where A n,k is defined by the following recurrence: A n,1 = A n,n = 1, A n+1 = k A n,k + (n + 2 k)a n,k 1, for2 k n The proof of Theorem 2 follows by induction. Since we have a formula for A n,k now, we will compute them for the first few values of n. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

49 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

50 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = 1 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

51 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = = 2 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

52 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = = = 6 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

53 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = = = = 24 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

54 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = = = = = 120 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

55 Like Pascal s Triangle, only not These do in fact match the coefficients we found earlier. Recall (or learn anew) that the numbers in the n th row of Pascal s Triangle sum to 2 n. Let s sum the rows of this triangle to see if we get anything interesting: 1 = = = = = 120 These are the factorials! Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

56 Eulerean numbers The A n,k s we defined earlier are, in fact, already well known in combinatorics as the Eulerean numbers, and they denote the number of n-permutations with k increasing runs. As a reminder, an n-permutation is a permutation of 1,2,,n. A definition of an increasing run is as follows: Increasing run For an n-permutation (s 1 s 2 s n ), a sequence of terms s i s i+1 s k where i k, s j < s j +1 for i j < k, and either k = n or s k > s k+1. Example: The permutation ( ) has the increasing runs (1 4), (3 5), and (2). Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

57 No calculus appears on this slide The patterns we noticed earlier are in fact properties of the Eulerean numbers: Theorem 3 For all n 1, (a) n k=1 A n,k = n!, (b) A n,k = A n,n+1 k for1 k n. Part (a) is obvious from the definition of A n,k. A proof of part (b) appears in [Advanced Combinatorics by L. Comtet, p. 242]. Recalling that a n = (1/2)h n (1/2), we can now say ( ) n 1 k=1 a n = A n,k(1/2) k n 2 (1 (1/2)) n+1 = A n,k 2 n k. k=1 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

58 Interpreting the formula The equation a n = n A n,k 2 n k has a natural interpretation for the lock k=1 combinations. If we take an n-permutation with k increasing runs, we can delimit the runs as seen for ( ): ( ). Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

59 Interpreting the formula The equation a n = n A n,k 2 n k has a natural interpretation for the lock k=1 combinations. If we take an n-permutation with k increasing runs, we can delimit the runs as seen for ( ): ( ). The most obvious combination we can get from this is ({4}, {1,3}, {2,5,6}), but we can also create other combinations by limiting any of the remaining spots. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

60 Interpreting the formula The equation a n = n A n,k 2 n k has a natural interpretation for the lock k=1 combinations. If we take an n-permutation with k increasing runs, we can delimit the runs as seen for ( ): ( ). The most obvious combination we can get from this is ({4}, {1,3}, {2,5,6}), but we can also create other combinations by limiting any of the remaining spots. This gives us 2 n k combinations for each n-permutation with k increasing runs, of which there are A n,k. Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

61 Interpreting the formula The equation a n = n A n,k 2 n k has a natural interpretation for the lock k=1 combinations. If we take an n-permutation with k increasing runs, we can delimit the runs as seen for ( ): ( ). The most obvious combination we can get from this is ({4}, {1,3}, {2,5,6}), but we can also create other combinations by limiting any of the remaining spots. This gives us 2 n k combinations for each n-permutation with k increasing runs, of which there are A n,k. A formula does exist for A n,k (see Comtet, pp. 243), we can combine it with the above formula to get ( ) n k 1 a n = ( 1) i n + 1 (k i ) n 2 n k. i k=1 i=0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

62 The Stirling numbers of the second kind Stirling number of the second kind For n N and 1 k n, the Stirling number of the second kind, denoted S(n,k) is equal to the number of ways to partition the set {1, 2,, n} into k unordered non-empty subsets. A combination lock is of course an ordered partition, and if we have k subsets, there are k! ways to order them. Hence, n a n = k!s(n,k). k=1 A formula for S(n,k) exists as well (Comtet, pp ): ( ) S(n,k) = 1 k 1 ( 1) n k (k i ) n. k! i i=0 Thus, we get our last formula for a n : ( ) n k 1 a n = ( 1) n k (k i ) n. i k=1 i=0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

63 Wrapping up Here are all of our formulas for a n : ( ) ( ) ( ) ( ) ( ) n n n n n n 1 a n = a n k = a n 1 + a n a 1 + k=1 k 1 2 n 1 n 2 a n = d n ( ) 1 dx n 2 e x x=0 3 a n = 1 2 k n k=0 2 k n! 4 a n 2(ln2) n+1 ( ) n k 1 5 a n = ( 1) i n + 1 (k i ) n 2 n k k=1 i=0 i ( ) n k 1 6 a n = ( 1) n k (k i ) n i k=1 i=0 a 0 Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35

64 References 1. L. Comtet, Advanced Combinatorics, D. Reidel, Dordrecht-Holland, D. Velleman and G. Call, Permutations and combination locks, Mathematics Magazine 68 (1995) Tim Sasaki (Western Oregon University) Combination Locks and Permutations April 9, / 35