Office/cafe hours: Tuesdays _and_ Thursdays, 10:50-11:50 in Van Vleck 813 Tuesdays, 2:45-4:00 at the State Street Steep and Brew Wednesdays, 1:30-3:30 at the State Street Steep and Brew READING For next Tuesday: read second section of Chapter 3 For next Thursday: read third section of Chapter 3. HOMEWORK Assignment #2: due next Thursday (see web) Remind students that I'm strict about deadlines. On each assignment, you must indicate how many hours you spent on each problem. This will help me to keep the problem sets from being too hard or too easy. We'll keep track of who isn't doing this and give a nominal (1%) penalty on your homework grade. If you didn’t work with anyone else (including me) you should write “I worked alone”. Reading the on-line solutions is important! Notice that I acknowledge students. One of them could be you next time around! Note answers at back of book: feel free to use them The problem set you've just done is pretty typical. This is not a class where you can do the homework by mimicking text illustrations; problems require thought, not memorization. Some problems will require you to go back to first principles. Sometimes the way to attack a problem is to explore: make up some concrete examples, see if what you're supposed to prove makes sense for those examples, understand why it's true, and then come up with a proof that holds in general. Or, if you can't solve a problem, think about a related but simpler problem. Thinking is good, but banging your head against a wall for hours isn't. So, if you've given a problem some real thought and haven't made any progress, ask me or a classmate for help. For info on the rules that govern this sort of cooperation, see the website (logistics). If you're having trouble, start seeking help early in the term. Collect homework Collect info sheets from new students. PIGEONHOLE PRINCIPLE Application 9: The Erdos-Szekeres Theorem Suppose that we are given a sequence of 17 real numbers a_1,...,a_17. Show that it contains an increasing subsequence of length 5 or a decreasing subsequence of length 5. My proof (“take two”): Suppose otherwise; that is, suppose that every increasing or decreasing subsequence of a_1,…,a_17 has length at most 4. For all k between 1 and 17, let {I(k) / D(k)} be the length of the longest {increasing / decreasing} subsequence whose first term is a_k. By our supposition, I(k) and D(k) are between 1 and 4 for all k between 1 and 17. Let the ordered pairs (1,1),(1,2),…,(4,1) be the PIGEONHOLES, and let the numbers 1,…,17 be the OBJECTS. Put the number k into the pigeonhole (I(k),D(k)). Since there are 17 objects and only 4x4 = 16 pigeonholes, two of the objects must be in the same pigeonhole. That is, there must exist i and j with 1i<j17 such that (I(i),D(i)) = (I(j),D(j)). But: if a_i a_j, then I(i) > I(j) (because we can always take the longest increasing sequence that starts with a_j and stick a_i at the front of it, obtaining an even longer increasing subsequence), whereas if a_i a_j, then D(i) > D(j) (for the same reason). Either way, we fail to have (I(i),D(i)) = (I(j),D(j)): contradiction! Alternatively: Use the strong form of the pigeonhole principle (as in Brualdi). Questions? ... Is 17 best possible? ... Note where the proof breaks down if 17 is replaced by 16. But note also that for proving that 17 is best possible, it’s not enough to show that some particular proof breaks down when 17 is replaced by 16. Why not? … So, is 17 best possible? … The sequence 4,3,2,1,8,7,6,5,12,11,10,9,16,15,14,13 has no increasing or decreasing subsequence of length greater than 4. To prove this last assertion, divide the 16 positions in the sequence into four quadruples, and note that (a) any increasing subsequence can contain at most element of each quadruplet, and (b) any decreasing subsequence must lie entirely within a single quadruplet. Combining (a) and the pigeonhole principle, we see that the sequence has no increasing subsequence of length > 4; and combining (b) and the pigeonhole principle, we see that the sequencehas no decreasing subsequence of length > 4. The Averaging Principle: Given a list of numbers, there is always a number on the list whose value is at least as large as the average value of all the numbers in the list. (Ditto for "at least as small as".) 200 children sit in a circle of 200 chairs. Some of the children are wearing blue, and the rest are wearing red. 100 of the chairs are blue, and the other 100 are red. Prove that there exists some k between 0 and 199 such that if each student moves k seats clockwise, at least half of the students will be sitting in chairs that match their outfits. Have you seen this problem before? … Application 8. Proof: For all k between 0 and 199, let f(k) be the number of children who will be in a matching chair if every student moves k seats clockwise. We want to show that for some k, f(k) ≥ 100. We will do this by proving that the average value of f(k) as k varies is ≥ 100. To this end, we use the “decomposition trick”, and write f(k) as f(k) = a_{k,1}+a_{k,2}+…+a{k,200}, where a_{k,i} is 1 if the ith child will be in a color-matched chair if he/she moves k seats clockwise, and a_{k,i} is 0 otherwise. What is the average value of a_{k,1} as k varies? … 1/2. What about a_{k,2}? … So what is the average value of f(k) as k varies? … 1/2 + 1/2 + … + 1/2 = (200)(1/2) = 100. Infinite combinatorics: Show that at least one of the digits 0,1,...,9 must occur infinitely often among the digits of pi. ... What are the pigeonholes? … The digits 0 through 9. What are the objects? … The numbers 1,2,3,… When does the object n go into the pigeonhole k? … When the nth digit of pi is k. Mention that this can't be proved for any digit in particular, with current methods! “What are your questions on Chapter 2? … Any typos or other mistakes?” VOTE ON DATE OF MIDTERM! The winner is: Tuesday, Mar. 9 (nobody voted by email) COUNTING Definitions: A union B = {x: x in A or x in B}, A intersect B = {x: x in A and x in B}, A \ B = {x: x in A but x not in B}, A x B = {(x,y): x in A and y in B}, |A| = number of elements in A. Addition Principle: For any two sets A and B, |A union B| = |A| + |B| - |A intersect B|. Consequently: |A union B| ≤ |A| + |B|, with equality precisely when A and B are disjoint. Subtraction Principle ("counting what you want by counting what you don't want"): For any set A contained in X, |X-A| = |X| - |A|. Inclusion-exclusion principle (simple case): If A and B are subsets of X, |X-(A union B)| = |X| - |A| - |B| + |A intersect B|.