# Strong Induction. Catalan Numbers.

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2 The game starts with a stack of n coins. In each move, you divide one stack into two nonempty stacks If the new stacks have height a and b, then you score ab points for the move. + you get 3 2 = 6 points What is the maximum score you can get? p. 2

3 10 coins The total score: = 45 points. Can we find a better strategy? p. 3

4 Theorem. Every way of unstacking n coins gives a score of S(n) = n(n 1) 2 points. And we want to prove it by induction. Let P(n) be the proposition that every way of unstacking n coins gives a score of S(n). In the inductive step we have to show that S(n) = S(k) + S(n k) + k(n k) = n(n 1). 2 We would like to have something a little stronger than ordinary induction. p. 4

5 Theorem. Every way of unstacking n coins gives a score of S(n) = n(n 1) 2 points. And we want to prove it by induction. Let P(n) be the proposition that every way of unstacking n coins gives a score of S(n). In the inductive step we have to show that S(n) = S(k) + S(n k) + k(n k) = n(n 1). 2 We would like to have something a little stronger than ordinary induction. p. 5

6 Theorem. Every way of unstacking n coins gives a score of S(n) = n(n 1) 2 points. And we want to prove it by induction. Let P(n) be the proposition that every way of unstacking n coins gives a score of S(n). In the inductive step we have to show that S(n) = S(k) + S(n k) + k(n k) = n(n 1). 2 We would like to have something a little stronger than ordinary induction. p. 6

7 Strong induction Principle of. Let P(n) be a predicate. If P(0) is true, and for all n, P(0), P(1),..., P(n) imply P(n + 1), then P(n) is true for all n. Strong induction allows you to assume P(0),..., P(n) in the inductive step, whereas in ordinary induction, you assume P(n) only. p. 7

8 Strong induction Strong induction is no more powerful than ordinary induction. Consider a predicate Q(n) = k 0 k n P(k) Q(n) says that P(k) is true for all 0 k n. Ordinary induction on the predicate Q(n) is equivalent to strong induction on P(n). Any theorem that can be proved with strong induction can also be proved with ordinary induction. However, an apeal to the strong induction principle can make some proofs a bit simpler. p. 8

9 Unstacking n coins Theorem. Every way of unstacking n coins gives a score of n(n 1) S(n) = points. 2 Proof. By strong induction. Let P(n) be the proposition that every way of unstacking n coins gives a score S(n). The base case: When n = 1, no moves is possible, so the score is S(1) = 0. The formula works, so P(1) is true. The inductive step: S(n) = S(k) + S(n k) + k(n k) k(k 1) (n k)(n k 1) = + + k(n k) 2 2 = 1 2 (k2 k + n 2 nk n nk + k 2 + k + 2kn 2k 2 ) = n(n 1). 2 p. 9

10 Consider a game in which two players take turns removing any positive number of matches they want from one of two piles of matches. The player who removes the last match wins the game. Show that if the two piles contain the same number of matches initially, the second player can always guarantee a win. p. 10

11 Let P(n) be the proposition that the second player has a winning strategy if each pile contains n matches. The base case: P(1) is true, because if each pile contains just 1 match, there is an obvious strategy for the second player. The inductive step: When n > 1. If the first player removes n matches from the first pile, the second player removes n matches from the other pile and wins. Otherwise, if the first player removes k < n from the first pile, the second player is doing the same, removing k matches from the other pile, so there are n k matches remain in both piles. And now, by the inductive hypothesis, for n k matches the second player has a winning strategy. p. 11

12 Let P(n) be the proposition that the second player has a winning strategy if each pile contains n matches. The base case: P(1) is true, because if each pile contains just 1 match, there is an obvious strategy for the second player. The inductive step: When n > 1. If the first player removes n matches from the first pile, the second player removes n matches from the other pile and wins. Otherwise, if the first player removes k < n from the first pile, the second player is doing the same, removing k matches from the other pile, so there are n k matches remain in both piles. And now, by the inductive hypothesis, for n k matches the second player has a winning strategy. p. 12

13 Count the number of good paths, C n Paths should go entirely below the diagonal line The number of such paths, C n, is the n th Catalan number. p. 13

14 Count the number of good paths, C n The cases when n is small: 0, 1, 2. C 0 = 1 C 1 = 1 C 2 = 2 p. 14

15 Recurrent formula C n is the number of paths that go below the diagonal (or touch the diagonal). Introduce D n, the number of paths that don t touch the diagonal in the middle points of the path. D n = C n 1 p. 15

16 Recurrent formula C n is the number of paths that go below the diagonal (or touch the diagonal). Introduce D n, the number of paths that don t touch the diagonal in the middle points of the path. D n = C n 1 p. 16

17 Recurrent formula (k, k) be the first point of the given path that is on the diagonal and k 0. Given (k, k), the number of paths is D k C n k p. 17

18 Recurrent formula The diagonal point can be anywhere: (1, 1), (2, 2),..., (n, n) So, to count the total number of paths, we add up these n cases: n C n = D 1 C n 1 + D 2 C n D n C 0 = D k C n k since D k = C k 1, we get C n = k=1 n C k 1 C n k k=1 p. 18

19 Recurrent formula The diagonal point can be anywhere: (1, 1), (2, 2),..., (n, n) So, to count the total number of paths, we add up these n cases: n C n = D 1 C n 1 + D 2 C n D n C 0 = D k C n k since D k = C k 1, we get C n = k=1 n C k 1 C n k k=1 p. 19

20 Recurrent formula n C n = C k 1 C n k k=1 p. 20

21 Closed form formula We already know that the number of paths from the bottom-left to the top-right corner is B n = 2n n Let s try to count the number of paths that cross the diagonal, there is B n C n of them. p. 21

22 Closed form formula Consider a bad path that crosses the diagonal. Lets say that the point P = (k, k + 1) is the first point above the diagonal. We mirror the remaining part of the path (shown in blue). We can construct such new path for any invalid path. p. 22

23 Closed form formula Since we mirror the path starting at P = (k, k + 1), the remaining part of the path consisted of (n k, n k 1) horizontal and vertical moves. Once reflected, it contains (n k 1, n k) moves. So, the resulting path ends up at the point Z = (k + n k 1, k n k) = (n 1, n + 1). It does not depend on k. p. 23

24 Closed form formula Every invalid paths becomes a path with (n 1, n + 1) horizontal and vertical moves. So there is n 1 + n + 1 2n B n C n = = n + 1 n + 1 of them. p. 24

25 Closed form formula Therefore, 2n C n = n 2n = n + 1 2n C n = B n n + 1 2n n 2n 2 n + 1 n = 1 n + 1 2n n p. 25

26 Three formulas for C n 2n C n = n C n = n C k 1 C n k k=1 2n n + 1 C n = 1 2n n + 1 n p. 26

27 Catalan numbers are more than that The number of ways to triangulate convex polygons: 1, 2, 5, 14,... p. 27

28 Catalan numbers are more than that Let s encode the path with bits, {0, 1}. If every move to the right is 1, and and every move up is 0: Well, not particularly interesting... p. 28

29 Catalan numbers are more than that Let s encode the path with parentheses, {(, )}. If every move to the right is (, and and every move up is ): Wyg ( ( ( ) ) ) ( ) ( ) p. 29

30 Catalan numbers are more than that ( ( ( ) ) ) ( ) ( ) C n is the number of strings made of n pairs of correctly balanced parentheses. p. 30

31 Catalan numbers are more than that C 2 = 2 C 3 = 5 C 4 = 14 C n is the number of full binary trees with n + 1 leaves: 2, 5, 14,... (A rooted binary tree is full if every internal node has two children) p. 31

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