# Discrete Mathematics: Homework 6 Due:

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1 Discrete Mathematics: Homework 6 Due: (3%) How many bit strings are there of length six or less? We use the sum rule, adding the number of bit strings of each length to 6. If we include the empty string, then we get = = 127 (using the formula for the sum of a geometric progression see Example 4 in Section 4.1). 2. (3%) How many strings are there of four lowercase letters that have the letter x in them? We can subtract from the number of strings of length 4 of lower case letters the number of string of length 4 of lower case letters other than x. Thus the answer is = 66, (6%) How many strings of four decimal digits (a) do not contain the same digit twice? (b) end with an even digit? (c) have exactly three digits that are 9s? (a) There are 10 ways to choose the first digit, 9 ways to choose the second, and so on; therefore the answer is =5040. (b) There 10 ways to choose each of the first three digits and 5 ways to choose the last; therefore the answer is = (c) There are 4 ways to choose the position that is to be different from 9, and 9 ways to choose the digit to go there. Therefore there are 4 9 = 36 such strings. 4. (5%) How many subsets of a set with 100 elements have more than one element? We know that there are subsets in all. Clearly 101 of them do not have more than one element, namely the empty set and the 100 sets consisting of 1 element. Therefore the answer is

2 Figure 1: Tree diagram 5. (5%) A palindrome is a string whose reversal is identical to the string. How many bit strings of length n are palindromes? The trick here is to realize that a palindrome of length n is completely determined by its first n/2 bits. This is true because once these bits are specified, the remaining bits, read from right to left, must be identical to the first n/2 bits, read from left to right. Furthermore, these first n/2 bits can be specified at will, and by the product rule there are 2 n/2 ways to do so. 6. (6%) Use a tree diagram to find the number of bit strings of length four with no three consecutive 0s. See Figure (4%) Let d be a positive integer. Show that among any group of d+1 (not necessarily

3 consecutive) integers there are two with exactly the same remainder when they are divided by d. There are only d possible remainders when an integer is divided by d, namely 0, 1,... d 1. By pigeonhole principle, if we have d + 1 remainders, then at least two must be the same. 8. (10%) (a) (7%)Show that if seven integers are selected from the first 10 positive integers, there must be at least two pairs of these integers with the sum 11. (b) (3%)Is the conclusion in part (a) true if six integers are selected rather than seven? (a) We can group the first ten positive integers into five subsets of two integers each, each subset adding up to 11: {1, 10}, {2, 9}, {3, 8}, {4, 7}, and {5, 6}. If we select seven integers from this set, then by the pigeonhole principle at least two of them come from the same subset. Furthermore, if we forget about these two in the same group, then there are five more integers and four groups; again the pigeonhole principle guarantees two integers in the same group. This gives two pairs of integers, each pair from the same group. In each case these two integers have a sum of 11, as desired. (b) No. The set {1, 2, 3, 4, 5, 6} has only 5 and 6 from the same group, so the only pair with sum 11 is 5 and (6%) How many numbers must be selected from the set {1, 3, 5, 7, 9, 11, 13, 15} to guarantee that at least one pair of these numbers add up to 16? We can apply the pigeonhole principle by grouping the numbers cleverly into pairs (subsets) that add up to 16, namely {1, 15}, {3, 13}, {5, 11}, and {7, 9}. If we select five numbers from the set {1, 3, 5, 7, 9, 11, 13, 15}, then at least two of them must fall within the same subset since there are only four subsets. Two numbers in the same subset are the desired pair that add up to 16. We also need to point out the choosing four numbers is not enough, since we could choose {1, 3, 5, 7}, and no

4 pair of them add up to more than (8%) How many bit strings of length 12 contain (a) exactly three 1s? (b) at most three 1s? (c) at least three 1s? (d) an equal number of 0s and 1s? (a) To specify a bit string of length 12 that contains exactly three 1s, we simply need to choose the three positions that contain the 1s. There are C(12, 3) = 220 ways to do that. (b) To contain at most three 1s means to contain three 1s, two 1s, one 1, or no 1s. Reasoning as in part (a), we see that there are C(12, 3) + C(12, 2) + C(12, 1) = = 299 such strings. (c) To contain at least three 1s means to contain three 1s, four 1s, five 1s, six 1s, seven 1s,..., 12 1s. We could reason as in part b, but we could have too many numbers to add. A simpler approach would be to figure out the number of ways not to have at least three 1s (i.e., to have two 1s, one 1, or no 1s) and then subtract that from 2 12, the total number of bit strings of length 12. This way we get 4096-( =4017). (d) To have an equal number of 0s and 1s in this case means to have six 1s. Therefore the answer is C(12, 6) = (2%) In how many ways can a set of two positive integers less than 100 be chosen? C(99, 2) = 99 98/2 = (9%) How many permutations of the letters ABCDEF GH contain (a) the string CDE?

5 (b) the strings AB, DE and GH? (c) the strings BCA and ABF? (a) If CDE is to be a substring, then we can think of that block of letters as super-letter, and the problem is to count permutations of six items - the letters A, B, F, G, H, and the super-letter CED. We see that the answer is P (6, 6) = 6! = 720. (b) This is similar to part (a). Glue AB into one item, glue DE into one item, and glue GH into one item, producing five items, so the answer is P (5, 5) = 5! = 120. (c) There are no permutations with both of these substrings, since B cannot be followed by both C and F at the same time. 13. (5%) Suppose that a department contains 10 men and 15 women. How many ways are there to form a committee with six members if it must have more women than men? C(15, 6)+C(15, 5)C(15, 1)+C(15, 4)C(15, 2) = = 96, (2%) What is the coefficient of x 8 x 9 in expansion of (3x + 2y) 17? ( 17 9 ) = = 81, 662, 929, (12%) Prove the identity ( n r )( r k) = ( n k)( n k r k ) whenever n, r, and k are nonnegative integers with r n and k r, (a) (8%)using a combinatorial argument. (b) (4%)using an argument based on the formula for the number of r-combinations of a set with n elements. (a) Suppose that we have a set with n elements, and we wish to choose a subset A with k elements and another, disjoint, subset with r k elements. The left-hand side gives us the number of ways to do this, namely the product of

6 the number of ways to choose the r elements that are to go into one or the other of the subsets and the number of ways to choose which of these elements are to go into the first of the subsets. The right-hand side gives us the number ways to go as well, namely the product of the number of ways to choose the first subset and the number of ways to choose the second subset from elements that remain. (b) One the one hand, ( n r )( r k) = n!/(r!(n r)!) r!/(k!(r k)! = n!/(k!(n r)!(r k)!) and on the other hand, ( n k)( n k r k ) = n!/(k!(n k)!) (n k)!/((r k)!(n r)!) = n!/(k!(n r)!(r k)!) 16. (12%) Show that if n is a positive integer, then ( 2n 2 ) = 2( n 2) + n 2 (a) (8%)using a combinatorial argument. (b) (4%)by algebraic manipulation. (a) To choose 2 people from a set of n men and n women, we can either choose 2 men (( n 2) ways to do so) or 2 women (( n 2) ways to do so) or one of each sex (n n ways to do so). Therefore the right-hand side counts the number of ways to do this (by the sum rule). The left-hand side counts the same thing, since we are simply choosing 2 people from 2n people. (b) 2( n 2) + n 2 = n(n 1) + n 2 = 2n 2 n = n(2n 1) = 2n(2n 1)/2 = ( 2n 2 )

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