Warm-up Simple methods Linear recurrences. Solving recurrences. Misha Lavrov. ARML Practice 2/2/2014

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1 Solving recurrences Misha Lavrov ARML Practice 2/2/2014

2 Warm-up / Review 1 Compute 100 k=2 ( 1 1 ) ( = 1 1 ) ( 1 1 ) ( 1 1 ). k Compute 100 k=2 ( 1 1 ) k 2. Homework: find and solve problem Algebra #7 from the February 2009 HMMT.

3 Warm-up / Review Solutions 1 Write 1 1 k k 1 as k. Then the product becomes Except for the 1 and the 100, every number occurs once in the numerator and once in the denominator, so the final answer is Write 1 1 k 2 ( as k2 1 k 2 ) ( = k 1 k k+1 k ) ( Then the product becomes ) ( ). 100 The second factor of each term is the reciprocal of the first factor of the next. Therefore the adjacent factors cancel, leaving only =

4 Three methods for solving recurrences Method 1: Guess and check Problem If a n = 2a n and a 0 = 0, find a formula for a n.

5 Three methods for solving recurrences Method 1: Guess and check Problem If a n = 2a n and a 0 = 0, find a formula for a n. 1 Find the first few values of a n : 0, 1, 3, 7, 15,... 2 Guess a formula: a n = 2 n 1. 3 We can prove this: { 2 n 1 = 2(2 n 1 1) + 1, = 0.

6 Three methods for solving recurrences Method 2: Convert to a sum Problem If a n = 2a n and a 0 = 0, find a formula for a n. 1 Divide by a suitable function for step 2 to work. a n 2 n = a n 1 2 n n. 2 Make a substitution to write the recurrence as s n = s n s n = s n n where s n = a n 2 n. 3 Solve for s n and reverse the substitution. n 1 s n = k = n a n = 2 n s n = 2 n 1. k=1

7 Three methods for solving recurrences Method 3: Convert to a product Problem If a n = 2a n and a 0 = 0, find a formula for a n. 1 Add or subtract something that makes step 2 work. a n + 1 = 2a n = 2(a n 1 + 1). 2 Make a substitution to write the recurrence as p n =? p n 1. p n = 2p n 1 where p n = a n Solve for p n and reverse the substitution. p n = 2 n p 0 = 2 n a n = p n 1 = 2 n 1.

8 Exercises 1 Solve the recurrences: x n = 2x n n, x 0 = 0. y n = 3y n 1 + 2n 1, y 0 = 0. z n = 3z 2 n 1, z 0 = 1. 2 In preparation for the next topic, solve the recurrence a n = a n 1 + 2a n 2 with a 0 = 2 and a 1 = 1.

9 Exercises Solutions: # 1 We solve x n by the method of sums. Dividing by 2 n, we get x n 2 n = x n 1 2 n This leads to the recurrence s n = s n 1 + 1, with s 0 = 0, where s n = x n /2 n. This is easy to solve: s n = n. Therefore x n = n 2 n.

10 Exercises Solutions: # 1 We solve x n by the method of sums. Dividing by 2 n, we get x n 2 n = x n 1 2 n This leads to the recurrence s n = s n 1 + 1, with s 0 = 0, where s n = x n /2 n. This is easy to solve: s n = n. Therefore x n = n 2 n. We solve y n by the method of products. Add n + 1 to both sides and factor to get y n + n + 1 = 3(y n 1 + n). This means that for p n = y n + n + 1 we have the recurrence p 0 = 1 and p n = 3p n 1, so p n = 3 n and therefore y n = 3 n n 1.

11 Exercises Solutions: # 1 We solve x n by the method of sums. Dividing by 2 n, we get x n 2 n = x n 1 2 n This leads to the recurrence s n = s n 1 + 1, with s 0 = 0, where s n = x n /2 n. This is easy to solve: s n = n. Therefore x n = n 2 n. We solve y n by the method of products. Add n + 1 to both sides and factor to get y n + n + 1 = 3(y n 1 + n). This means that for p n = y n + n + 1 we have the recurrence p 0 = 1 and p n = 3p n 1, so p n = 3 n and therefore y n = 3 n n 1. Finally, for z n, we take the log of both sides. The recurrence z n = 3z 2 n 1 becomes log 3 z n = 2 log 3 z n 1 + 1, with log 3 z 0 = 0. This is the recurrence we took great pains to solve earlier, so log 3 z n = 2 n 1, and therefore z n = 3 2n 1.

12 Exercises Solutions: # 2 One way to approach the two-term recurrence is to begin with the method of products. Add a n 1 to both sides; then a n + a n 1 = 2a n 1 + 2a n 2 = 2(a n 1 + a n 2 ). If p n = a n + a n 1, we have p n = 2p n 1, with p 1 = 3. Therefore p n = 3 2 2n. However, this doesn t solve the problem fully: a formula for p n only tells us that a n = a n n.

13 Exercises Solutions: # 2 One way to approach the two-term recurrence is to begin with the method of products. Add a n 1 to both sides; then a n + a n 1 = 2a n 1 + 2a n 2 = 2(a n 1 + a n 2 ). If p n = a n + a n 1, we have p n = 2p n 1, with p 1 = 3. Therefore p n = 3 2 2n. However, this doesn t solve the problem fully: a formula for p n only tells us that a n = a n n. We can solve this by the method of sums. Let s n = ( 1) n a n ; then we have the recurrence s n = s n ( 2)n, with s 0 = 2. Therefore s n = n k=1 ( 2)k = ( 2) n + 1. Finally, because a n = ( 1) n s n, we get a n = 2 n + ( 1) n.

14 Solving linear recurrences Recurrences such as a n = a n 1 + 2a n 2 come up so often there is a special method for dealing with these. 1 Find all solutions of the form a n = r n. We will need to solve a quadratic equation, since r n = r n 1 + 2r n 2 just means r 2 = r + 2. In this case, the solutions are r = 2 and r = 1. 2 Find a combination of these solutions that satisfies the initial conditions. Here, if a n = x 2 n + y ( 1) n, we have { x y ( 1) 0 = a 0 = 2, So a n = 2 n + ( 1) n. x y ( 1) 1 = a 1 = 1. In general, if a n is a linear combination of a n 1,..., a n k, we d have to solve a degree-k polynomial to get what we want.

15 Example of solving linear recurrences The Fibonacci numbers are defined by F 0 = 0, F 1 = 1, F n = F n 1 + F n 2. Find a formula for F n.

16 Example of solving linear recurrences The Fibonacci numbers are defined by F 0 = 0, F 1 = 1, F n = F n 1 + F n 2. Find a formula for F n. 1 To find the exponential solutions, we solve r 2 = r + 1 to get r = 1 ± 5. Call these two solutions φ 1 and φ 2. 2

17 Example of solving linear recurrences The Fibonacci numbers are defined by F 0 = 0, F 1 = 1, F n = F n 1 + F n 2. Find a formula for F n. 1 To find the exponential solutions, we solve r 2 = r + 1 to get r = 1 ± 5. Call these two solutions φ 1 and φ Now we know F n = xφ n 1 + yφn 2. To find x and y, we solve: { { F 0 = xφ yφ0 2, x + y = 0, F 1 = xφ yφ1 2. ( ) ( ) x y = 1.

18 Example of solving linear recurrences The Fibonacci numbers are defined by F 0 = 0, F 1 = 1, F n = F n 1 + F n 2. Find a formula for F n. 1 To find the exponential solutions, we solve r 2 = r + 1 to get r = 1 ± 5. Call these two solutions φ 1 and φ Now we know F n = xφ n 1 + yφn 2. To find x and y, we solve: { { F 0 = xφ yφ0 2, x + y = 0, F 1 = xφ yφ1 2. ( ) ( ) x y = 1. 3 So x = 1 5, y = 1 5, and F n = ( ) n ( ) n 5.

19 One thing that can go wrong Consider the recurrence a n = 4a n 1 4a n 2, with a 0 = 0 and a 1 = 2. What goes wrong? Try to solve this using our previous methods.

20 One thing that can go wrong Consider the recurrence a n = 4a n 1 4a n 2, with a 0 = 0 and a 1 = 2. What goes wrong? Try to solve this using our previous methods. The problem is that r 2 = 4r 4 only has one solution: r = 2. But multiples of 2 n aren t enough to fit the initial conditions.

21 One thing that can go wrong Consider the recurrence a n = 4a n 1 4a n 2, with a 0 = 0 and a 1 = 2. What goes wrong? Try to solve this using our previous methods. The problem is that r 2 = 4r 4 only has one solution: r = 2. But multiples of 2 n aren t enough to fit the initial conditions. Writing a n 2a n 1 = 2(a n 1 2a n 2 ) yields the one-term recurrence a n = 2a n n. We have already solved this today; the solution is a n = n 2 n. In general, when a root r is repeated k times, it yields the solutions a n = r n, a n = n r n,..., a n = n k 1 r n, and we solve for their combination as usual. (Here, all solutions to the recurrence are a combination of 2 n and n 2 n.)

22 Exercises 1 Solve the recurrence x n = 2x n 1 + x n 2 with x 0 = x 1 = 1. 2 Solve the recurrence y n = y n 1 + 2y n with y 0 = 0 and y 1 = 1. 3 Solve the recurrence z n = z n 1 + z n 2 z n 3 with z 0 = 1, z 1 = 0, and z 2 = 3.

23 Exercises Solutions 1 The characteristic equation is r 2 = 2r + 1, which yields r = 1 ± 2. Determining the constants, we see that x n = (1 + 2) n + (1 2) n. 2 2 By a trick analogous to the method of products, we write y n + 1 = (y n 1 + 1) + 2(y n 2 + 1). Then y n + 1 is some combination of 2 n and ( 1) n ; from y = 1 and y = 2, we can deduce y n + 1 = 2 n, so y n = 2 n 1. 3 Here we must first solve the cubic r 3 = r 2 + r 1, whose three roots are 1, 1, 1. This means z n is a combination of 1, n, and ( 1) n. From the initial conditions, we see that z n = n + ( 1) n.

24 More math problems Inspired by 1990 VTRMC, #6. The sequence (y n ) obeys the recurrence y n = y n 1 (2 y n 1 ). Solve for y n in terms of y AIME II, #11. For a positive integer m, let a 0, a 1,..., a m be a sequence such that a 0 = 37, a 1 = 72, a k+1 = a k 1 3 a k for k = 1,..., m 1, and finally, a m = 0. Find m PUMαC, Algebra A #7. Two sequences x n and y n are defined by x 0 = y 0 = 7 and { x n = 4x n 1 + 3y n 1 y n = 3y n 1 + 2x n 1. Find lim n x n y n. [Or just solve for x n and y n.] 2011 VTRMC, #2. The sequence (a n ) is defined by a 0 = 1, a 1 = 0, and a n = a 2 n 1 n2 a n 2 1. Find a 100.

25 More math problems Solutions 1990 VTRMC, #6. Rewrite the recurrence as 1 y n = (1 y n 1 ) 2. It follows that 1 y n = (1 y 0 ) 2n, so y n = 1 (1 y 0 ) 2n AIME II, #11. Let b k = a k a k+1. From the recurrence, we have b k = b k 1 3, with b 0 = So b k = k. We have b 888 = a 888 a 889 = 0; but a because b 887 = a 887 a 888 = 3. Therefore m = PUMαC, Algebra A #7. Note that x n y n = 2x n 1, so we can write x n as 7x n 1 6x n 2. Solving this, we get x n = n 7 5, so y n = n + 7 xn 5. In the limit, y n 42/5 28/5 = VTRMC, #2. From computing the first few terms, we guess that a n = n 2 1, which is confirmed by induction: n 2 1 = ((n 1) 2 1) 2 n 2 ((n 2) 2 1) 1.