# Recurrence Relations

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1 Recurrence Relations Time complexity for Recursive Algorithms Can be more difficult to solve than for standard algorithms because we need to know complexity for the sub-recursions of decreasing size Recurrence relations give us a powerful tool for calculating exact time complexities including constant factors A Recurrence relation is a function t(n) defined in terms of previous values of n For discrete time steps When time is continuous we use differential equations (another course) t(n) = a t(n-1) Also use notation t n = at n-1 or t(n+1) = at(n) Want to derive closed form (equation that does not refer to other t i ) CS Divide and Conquer/Recurrence Relations 1

2 Factorial(n) if n=0 return 1 else return Factorial(n-1) n Factorial Example Complexity recurrence relation: C(n) = C(n-1) + 3 n is a number with a max, not length, assume order 1 multiply What is the generated complexity sequence Must know the initial condition: C(0) = 1 Could build a sequence or table of the values How long to compute C(n) from sequence Better if we can find a closed form equation rather than a recurrence Which would be what in this case? CS Divide and Conquer/Recurrence Relations 2

3 Towers of Hanoi Example Example: Tower of Hanoi, move all disks to third peg without ever placing a larger disk on a smaller one. Use a strategy to decide time complexity without having to work out all the algorithmic details CS Divide and Conquer/Recurrence Relations 3

4 Towers of Hanoi Example Example: Tower of Hanoi, move all disks to third peg without ever placing a larger disk on a smaller one. C(n) = C(n-1) +... CS Divide and Conquer/Recurrence Relations 4

5 Towers of Hanoi Example Example: Tower of Hanoi, move all disks to third peg without ever placing a larger disk on a smaller one. C(n) = C(n-1) CS Divide and Conquer/Recurrence Relations 5

6 Towers of Hanoi Example Example: Tower of Hanoi, move all disks to third peg without ever placing a larger disk on a smaller one. C(n) = C(n-1) C(n-1) CS Divide and Conquer/Recurrence Relations 6

7 Towers of Hanoi Example Example: Tower of Hanoi, move all disks to third peg without ever placing a larger disk on a smaller one. C(n) = C(n-1) C(n-1) C(n) = 2C(n-1) + 1 C(1) = 1 CS Divide and Conquer/Recurrence Relations 7

8 Tower of Hanoi Example Given C(n) = 2C(n-1) + 1 and the initial condition C(1) = 1 What is the complexity for an arbitrary value of n (i.e., a closed form)? Could build a table and see if we can recognize a pattern For more complex problems we will not be able to find the closed form by simple examination and we will thus need our solution techniques for recurrence relations Note that we can get basic complexity without having yet figured out all the details of the algorithm CS Divide and Conquer/Recurrence Relations 8

9 Recurrence Relations Example Recurrence Relation: End of day growth in a savings account In terms of interest, deposits, and withdrawals CS Divide and Conquer/Recurrence Relations 9

10 Recurrence Relations Most general form for our purposes t(n) + f(t(n-1), t(n-2),..., t(n-k)) = g(n) or t(n+k) + f(t(n+k-1), t(n+k-2),..., t(n)) = g(n+k) A recurrence relation is said to have order k when t(n) depends on up to the k previous values of n in the sequence We will work with the linear form which is a 0 t(n) + a 1 t(n-1) a k t(n-k) = g(n) a i are the coefficients of the linear equation Coefficients could vary with time: a i (n) g(n) is the forcing function CS Divide and Conquer/Recurrence Relations 10

11 Recurrence Order What are the orders and differences between the following? a 0 t(n) = a 1 t(n-1) + a 2 t(n-2) a 0 t(n+2) - a 1 t(n+1) a 2 t(n) = 0 b 0 y(w) b 2 y(w-2) = b 1 y(w-1) Change of variables: let n = w and t = y and a = b What are the order of these? t(n) = a 1 t(n-1) + a 3 t(n-3) 0 valued coefficients t(n) = a 1 t(n-1) + a 3 t(n-3) + g(n) Order is independent of forcing function t(n-1) = a 1 t(n-2) + a 3 t(n-4) CS Divide and Conquer/Recurrence Relations 11

12 Homogeneous Linear Recurrence Relations with Constant Coefficients If coefficients do not depend on time (n) they are constant coefficients When linear these are also called LTI (Linear time invariant) If the forcing term/function is 0 the equation is homogenous This is what we will work with first (Homogenous LTI) a 0 t(n) + a 1 t(n-1) a k t(n-k) = 0 A solution of a recurrence relation is a function t(n) only in terms of the current n that satisfies the recurrence relation CS Divide and Conquer/Recurrence Relations 12

13 Fundamental Theorem of Algebra For every polynomial of degree n, there are exactly n roots You spent much of high school solving these where you set the equation to zero and solved the equation by finding the roots Roots may not be unique CS Divide and Conquer/Recurrence Relations 13

14 Solving Homogenous LTI RRs Assume RR: t n - 5t n-1 + 6t n-2 = 0 Do temporary change of variables: t n = r n for r 0 This gives us the Characteristic Equation r n - 5r n-1 + 6r n-2 = 0 Multiply by r n-2 /r n-2 to get r n-2 (r 2-5r+6) = 0 Since r 0 we can divide out the r n-k term. Thus could just initially divide by r n-k where k is the order of the homogenous equation (r 2-5r+6) = 0 = (r-2)(r-3), This gives us roots of 2 and 3 Substituting back t n = r n gives solutions of 2 n or 3 n Show that 3 n is a solution (root) for the recurrence relation CS Divide and Conquer/Recurrence Relations 14

15 Solving Homogenous LTI RRs Assume RR: t n - 5t n-1 + 6t n-2 = 0 Do temporary change of variables: t n = r n for r 0 This gives us the Characteristic Equation r n - 5r n-1 + 6r n-2 = 0 Multiply by r n-2 /r n-2 to get r n-2 (r 2-5r+6) = 0 Since r 0 we can divide out the r n-k term. Thus could just initially divide by r n-k where k is the order of the homogenous equation (r 2-5r+6) = 0 = (r-2)(r-3), This gives us roots of 2 and 3 Substituting back t n = r n gives solutions of 2 n or 3 n The general solution is all linear combinations of these solutions Thus the general solution is t n = c 1 3 n + c 2 2 n Any of these combinations is a solution. You will show this in your homework 15

16 Specific Solutions The general solution represents the infinite set of possible solutions, one for each set of initial conditions Given a general solution such as t n = c 1 3 n + c 2 2 n the specific solution with specific values for c 1 and c 2 depends on the specific initial conditions Existence and Uniqueness: There is one and only one solution for each setting of the initial conditions Since the original recurrence was 2 nd order we need initial values for t 0 and t 1 Given these we can solve m equations with m unknowns to solve for the coefficients c 1 and c 2 If t 0 = 2 and t 1 = 3 what is the specific solution for t n - 5t n-1 + 6t n-2 = 0 CS Divide and Conquer/Recurrence Relations 16

17 Roots of Multiplicity Assume a situation where the characteristic function has solution (r-1)(r-3) 2 = 0 The equation has a root r (=3) of multiplicity 2 To maintain linear independence of terms, for each root with multiplicity j we add the following terms to the general solution t n = r n, t n = nr n, t n = n 2 r n,..., t n = n j-1 r n General solution to (r-1)(r-3) 2 = 0 is t n = c 1 1 n + c 2 3 n + nc 3 3 n CS Divide and Conquer/Recurrence Relations 17

18 Solving Homogenous LTI RRs 1. Set t n = r n for r 0 to get the Characteristic Equation 2. Divide by r n-k 3. Solve for roots 4. Substitute back t n = r n to get solutions 5. The general solution is all linear combinations of these solutions 6. Use initial conditions to get the exact solution t n - 5t n-2 = -4t n-1 Initial Conditions: t 0 = 1 and t 1 = 2 CS Divide and Conquer/Recurrence Relations 18

19 Solving Homogenous LTI RRs t n + 4t n-1-5t n-2 = 0 Initial Conditions: If t 0 = 1 and t 1 = 2 Solutions are (-5) n and 1 n General solution is t n = c 1 + c 2 (-5) n Exact solution is t n = 7/6+ -1/6(-5) n CS Divide and Conquer/Recurrence Relations 19

20 Solutions to Non-Homogeneous LTI RRs General form - no general solution a 0 t(n) + a 1 t(n-1) a k t(n-k) = g(n) We will solve a particular and common form with a geometric forcing function a 0 t(n) + a 1 t(n-1) a k t(n-k) = b n p(n) How to solve Brute force - manipulate it until it is a homogeneous recurrence relation and then solve for the roots as we have just discussed Use a convenient shortcut which we will introduce Example: t n - 3t n-1 = 4 n (note b = 4 and p(n) = 1) CS Divide and Conquer/Recurrence Relations 20

21 Brute Force Example t n - 3t n-1 = 4 n To become homogenous, all terms must be in terms of t. Can get rid of 4 n term by finding two versions equal to 4 n-1 t n-1-3t n-2 = 4 n-1 (change of variable, replaced n with n-1) t n /4-3/4t n-1 = 4 n-1 (start from initial RR and divide by 4) t n-1-3t n-2 = t n /4-3/4t n-1 t n /4-7/4t n-1 + 3t n-2 = 0 (set them equal) (Homogeneous RR) r n /4-7/4r n-1 + 3r n-2 = 0 (Characteristic function) r 2 /4-7/4r + 3 = 0 (Divide by r n-2 - remember r 0) r 2-7r + 12 = 0 (Multiply both sides by 4) (r -3)(r-4) = 0 t n = c 1 3 n + c 2 4 n (General solution to the recurrence) CS Divide and Conquer/Recurrence Relations 21

22 An Easier Way: Shortcut Rule a 0 t n + a 1 t n a k t n-k = b n p(n) can be transformed to (a 0 r k + a 1 r k a k )(r - b) d+1 = 0 which is a homogeneous RR where d is the order of polynomial p(n) and k is the order of the recurrence relation Same example: t n - 3t n-1 = 4 n using rule where d = 0 and k = 1, RR is transformed to (r 1-3)(r - 4) 1 = 0 t n = c 1 3 n + c 2 4 n (Same general solution to the recurrence) CS Divide and Conquer/Recurrence Relations 22

23 Example using Shortcut Rule a 0 t n + a 1 t n a k t n-k = b n p(n) can be transformed to (a 0 r k + a 1 r k a k )(r - b) d+1 = 0 where d is the order of polynomial p(n) Another example: t n - 3t n-1 = 4 n (2n + 1) CS Divide and Conquer/Recurrence Relations 23

24 More on Shortcut Rule a 0 t n + a 1 t n a k t n-k = b n p(n) can be transformed to (a 0 r k + a 1 r k a k )(r - b) d+1 = 0 Another example: t n - 3t n-1 = 4 n (2n + 1) using rule where d = 1 and k = 1, RR is transformed to (r 1-3)(r - 4) 2 = 0 4 is a root of multiplicity 2 t n = c 1 3 n + c 2 4 n + c 3 n4 n (general solution to the recurrence) Need three initial values to find a specific solution - why? Would if we're only given one initial value (i.e. t 0 = 0), which is probable since the original RR is order 1 Can pump RR for as many subsequent initial values as we need: t 1 =?, t 2 =? CS Divide and Conquer/Recurrence Relations 24

25 More on Shortcut Rule a 0 t n + a 1 t n a k t n-k = b n p(n) can be transformed to (a 0 r k + a 1 r k a k )(r - b) d+1 = 0 Another example: t n - 3t n-1 = 4 n (2n + 1) using rule where d = 1 and k = 1, RR is transformed to (r 1-3)(r - 4) 2 = 0 4 is a root of multiplicity 2 t n = c 1 3 n + c 2 4 n + c 3 n4 n (general solution to the recurrence) Need three initial values to find a specific solution - why? Would if we're only given one initial value (i.e. t 0 = 0), which is probable since the original RR is order 1 Can pump RR for as many subsequent initial values as we need: t 1 = 12, t 2 = 116 Another example: 2t n - 3t n-1 + t n-2 = n CS Divide and Conquer/Recurrence Relations 25

26 Tower of Hanoi Revisited t(n) = 2t(n-1) + 1 t(1) = 1 Solve the Recurrence Relation What kind of RR is it? CS Divide and Conquer/Recurrence Relations 26

27 Tower of Hanoi Revisited t(n) = 2t(n-1) + 1 t(1) = 1 Solve the Recurrence Relation a 0 t n + a 1 t n a k t n-k = b n p(n) can be transformed to (a 0 r k + a 1 r k a k )(r - b) d+1 = 0 which is a homogeneous RR where d is the order of polynomial p(n) and k is the order of the recurrence relation CS Divide and Conquer/Recurrence Relations 27

28 Divide and Conquer Recurrence Relations and Change of Variables Most divide and conquer recurrence relations are of the form t(n) = a t(n/b) + g(n) These are not recurrence relations like we have been solving because they are not of finite order Not just dependent on a predictable set of t(n-1)... t(n-k), but the arbitrarily large difference t(n/b) with a variable degree log b n We can often use a change of variables to translate these into the finite order form which we know how to deal with CS Divide and Conquer/Recurrence Relations 28

29 Change of Variables - Binary Search Example Example (binary search): T(n) = T(n/2) + 1 and T(1) = 1 Replace n with 2 k (i.e. Set 2 k = n) Can do this since we assume n is a power of 2 Thus k = log 2 n In general replace n with b k assuming n is a power of b and thus k = log b n With change of variable: T(2 k ) = T(2 k /2) + 1 = T(2 k-1 ) + 1 One more change of variable: Replace T(b k ) with t k Then: T(2 k ) = T(2 k-1 ) + 1 becomes t k = t k Now we have a non-homogeneous linear recurrence which we know how to solve CS Divide and Conquer/Recurrence Relations 29

30 Change of Variables Continued t k = t k transforms to t k - t k-1 = 1 k k 0 by non-homogeneous formula (with b=1 and d=0) transforms to (r-1)(r-1) root 1 of multiplicity 2 t k = c 1 1 k + k c 2 1 k = c 1 + c 2 k General solution under change of variables First, re-substitute T(b k ) for t k T(2 k ) = c 1 + c 2 k Second, re-substitute n for b k and log b n for k T(n) = c 1 + c 2 log 2 n General solution under original variables CS Divide and Conquer/Recurrence Relations 30

31 Change of Variables Completed T(n) = c 1 + c 2 log 2 n Need specific solution with T(1) = 1 a given Pump another initial condition from original recurrence relation T(n) = T(n/2) + 1 which is T(2) = T(2/2) + 1 = 2 T(n) = log 2 n + 1 Specific solution T(n) = O(log 2 n) = O(logn) 1 = c 1 + c 2 log 2 1 = c 1 + c = c 1 + c 2 log 2 2 = c 1 + c 2 1 c 1 = c 2 = 1 CS Divide and Conquer/Recurrence Relations 31

32 Change of Variables Summary To solve recurrences of the form T(n) = T(n/b) + g(n) 1. Replace n with b k (assuming n is a power of b) 2. Replace T(b k ) with t k 3. Solve as a non-homogenous recurrence (e.g. shortcut rule) 4. In the resulting general solution change the variables back a) Replace t k with T(b k ) b) Replace b k with n and replace k with log b n 5. Using this finalized general solution, use the initial values to solve for constants to obtain specific solution Remember values of n must be powers of b Note unfortunate overloaded use of b and k in the shortcut In initial recurrence, b is task size divider, and k is the order index In shortcut, b is the base of the forcing function, and k is the order of the transformed recurrence CS Divide and Conquer/Recurrence Relations 32

33 Change of Variables Summary To solve recurrences of the form T(n) = T(n/b) + g(n) 1. Replace n with b k (assuming n is a power of b) 2. Replace T(b k ) with t k 3. Solve as a non-homogenous recurrence (e.g. shortcut rule) 4. In the resulting general solution change the variables back a) Replace t k with T(b k ) b) Replace b k with n and replace k with log b n 5. Using this finalized general solution, use the initial values to solve for constants to obtain specific solution Remember values of n must be powers of b Do example: T(n) = 10T(n/5) + n 2 with T(1) = 0 where n is a power of 5 (1, 5, 25 ) CS Divide and Conquer/Recurrence Relations 33

34 Note that is not necessary to Rewrite the logs for convenience. It is just as easy (perhaps easier) to leave them as they are in doing the steps, leading to an equivalent final solution of (5/3)10 log 5 n + (5/3)25 log 5 n

35 More Change of Variables Change of variables can be used in other contexts to transform a recurrence nt(n) = (n-1)t(n-1) + 3 for n > 0 Not time invariant since after dividing by n the second coefficient is (n-1)/n Can do a change of variables and replace nt(n) with t n Get t n = t n Now it is a simple non-homogenous RR which we can solve and then change back the variables You get to do one like this for your homework CS Divide and Conquer/Recurrence Relations 35

36 Divide and Conquer - Mergesort Sorting is a natural divide and conquer algorithm Merge Sort Recursively split list in halves Merge together Real work happens in merge - O(n) merge for sorted lists compared to the O(n 2 ) required for merging unordered lists Tree depth log b n Complexity - O(nlogn) 2-way vs 3-way vs b-way split? What is complexity of recurrence relation? Master theorem Exact solution (Your homework) CS Divide and Conquer Applications 36

37 Quicksort Mergesort is Θ(nlogn), but inconvenient for implementation with arrays since we need space to merge Quicksort sorts in place, using partitioning Example: Pivot about a random element (e.g. first element (3)) Starting next to pivot, swap the first element from left that is > pivot with first element from right that is pivot For last step swap pivot with last swapped digit pivot before after At most n swaps Pivot element ends up in it s final position No element left or right of pivot will flip sides again Sort each side independently Recursive Divide and Conquer approach Complexity of Quicksort? CS Divide and Conquer Applications 37

38 Quicksort Similar divide and conquer philosophy Recurse around a random pivot Pros and Cons In place algorithm - do not need extra memory Speed depends on how well the random pivot splits the data Random, First as pivot?, median of first middle and last Worst case is O(n 2 ) Average case complexity is still O(nlogn) but has better constant factors than mergesort On average swaps only n/2 elements vs merge which moves all n with each merge, but deeper Empirical Analysis later For Quicksort the work happens at partition time before the recursive call (O(n) at each level to pivot around one value), while for mergesort the work happens at merge time after the recursive calls. The last term in the master theorem recurrence includes both partition and combining work. CS Divide and Conquer Applications 38

39 Selection and Finding the Median Median is the 50th percentile of a list The middle number If even number, then take the average of the 2 middle numbers Book suggests taking the smallest of the two Median vs Mean Summarizing a set of numbers with just one number Median is resistant to outliers - Is this good or bad? Algorithm to find median? CS Divide and Conquer Applications 39

40 Selection and Finding the Median Median is the 50th percentile of a list The middle number If even number, then take the average of the 2 middle numbers Book suggests taking the smallest of the two Median vs Mean Summarizing a set of numbers with just one number Median is resistant to outliers - Is this good or bad? Algorithm to find median Sort set S and then return the middle number - nlogn Faster approach: Use a more general algorithm: Selection(S,k) Finds the k th smallest element of a list S of size n Median: Selection(S,floor(n/2)) How would you find Max or Min using Selection? CS Divide and Conquer Applications 40

41 Selection Algorithm S = {2, 36, 5, 21, 8, 13, 15, 11, 20, 5, 4, 1} To find Median call Selection(S,floor( S /2)) = Selection(S,6) Let initial random pivot be v = 5 How do we pick the pivot? - more on that in a minute Compute 3-way split which is O(n) (like a pivot in Quicksort): S L = {2, 4, 1} S v = {5, 5} S R = {36, 21, 8, 13, 15, 11, 20}! selection( SL, k) if k SL # selection( S, k) = \$ v if S < k S + S # & selection( SR, k ( SL + Sv )) if k > SL + Sv L L v Would if initial random pivot had been 13? Which branch would we take? CS Divide and Conquer Applications 41

42 Best case Best Case Selection Complexity Pick the median value as the pivot (or close to it) each time so that we cut the list in half at each level of the recursion Of course if we could really pick the median value then we wouldn't need selection to find the median If we can split the list close to half each time, then: T(n) = T(n/2) + O(n) a = 1 since since just recurse down 1 branch each time Note that just like quicksort, O(n) work is done before the recursive call and that no combining work need be done after the recursion threshold. Just the opposite of mergesort, etc. By master theorem best case complexity is O(n) because time dominated by root node CS Divide and Conquer Applications 42

43 Given: Master Theorem t(n) = at( " n /b#) + O(n d ) Where a > 0, b > 1, d 0 and a = number of sub-tasks that must be solved n = original task size (variable) n/b = size of sub-instances d = polynomial order of partitioning/recombining cost Then: # O(n d ) % t(n) = \$ O(n d logn) % O(n log b & a ) if a < b d d > log b a if a = b d d = log b a if a > b d d < log b a This theorem gives big O complexity for most common DC algorithms CS Divide and Conquer/Recurrence Relations 43

44 Average/Worst Case Complexity But can we pick the Median? Pick Random instead Best case - Always pick somewhat close to the median - O(n) Worst case complexity Always happen to pick smallest or largest element in list Then the next list to check is of size n-1 with an overall depth of n n + (n-1) + (n-2) +... = O(n 2 ) Exact same issue as with Quicksort CS Divide and Conquer Applications 44

45 Average Case Complexity Average case complexity Assume a good pivot is one between the 25 th and 75 th percentile, these divide the space by at least 25% Chance of choosing a good pivot is 50% - coin flip On average "heads" will occur within 2 flips Thus, on average divide the list by 3/4ths rather than the optimal 1/2 T(n) = T(3n/4) + O(n) Complexity? CS Divide and Conquer Applications 45

46 Average Case Complexity Average case complexity Assume a good pivot is one between the 25 th and 75 th percentile, these divide the space by at least 25% Chance of choosing a good pivot is 50% - coin flip On average "heads" will occur within 2 flips Thus, on average divide the list by 3/4ths rather than the optimal 1/2 T(n) = T(3n/4) + O(n) Complexity? This is still O(n), b = 4/3 Note that as long as a=1, b>1 and d=1, then work is decreasing, and the complexity will be dominated by the work at the root node which for this case is O(n) t(n) = at( " n /b#) + O(n d ) CS Divide and Conquer Applications 46

47 Best Case Complexity Intuition How much work at first level for optimal selection? - O(n) How much work at next level? - O(n/2) Doesn't that feel like an nlogn? Remember geometric series (HW# 0.2) for c > 0 f(n) = 1 + c + c c n = (1-c n+1 )/(1-c) if c < 1 then f = Θ(1), which is the first term Since, 1 < f(n) = (1-c n+1 )/(1-c) < 1/(1-c) So, 1 + 1/2 + 1/ /2 n < 2 Selection: n + n/2 + n/4... 1/2 n = n(1 + 1/2 + 1/ /2 n ) < 2n So what is bound for c = 3/4 (b = 4/3)? CS Divide and Conquer Applications 47

48 Matrix Multiplication One of the most common time-intensive operations in numerical algorithms Thus any improvement is valuable What is complexity of the standard approach? = ( ) ( ) ( ) ( ) = CS Divide and Conquer Applications 48

49 Matrix Multiplication One of the most common time-intensive operations in numerical algorithms Thus any improvement is valuable What is complexity of the standard approach? An n by n matrix has n 2 elements and each element of the product of 2 matrices requires n multiplies O(n 3 ) = ( ) ( ) ( ) ( ) = CS Divide and Conquer Applications 49

50 Divide and Conquer Matrix Multiplication X Y = A B C D E F G H = AE + BG CE + DG AF + BH CF + DH A thru H are sub-block matrices This subdivides the initial matrix multiply into 8 matrix multiplies each of half the original size T(n) = 8T(n/2) + O(n 2 ) - The O(n 2 ) term covers the matrix additions of the O(n 2 ) terms in the matrices Complexity for this recurrence is: O(n log 2 8 ) = O(n 3 ) However, we can use a trick similar to the Gauss multiply trick in our divide and conquer scheme CS Divide and Conquer Applications 50

51 Strassen's Algorithm In 1969 Volkler Strassen discovered that: a b c d e f g h = m 2 +m 3 m 1 +m 2 +m 5 +m 6 m 1 +m 2 +m 4 -m 7 m 1 +m 2 +m 4 +m 5 m 1 = (c + d - a) (h f + e) m 2 = (a e) m 3 = (b g) m 4 = (a - c) (h - f) m 5 = (c + d) (f - e) m 6 = (b - c + a - d) h m 7 = d (e + h f - g) Now we have 7 matrix multiplies rather than 8 T(n) = 7T(n/2) + O(n 2 ) which gives O(n log 2 7 ) O(n 2.81 ) Even faster similar approaches have recently been shown CS Divide and Conquer Applications 51

52 Divide and Conquer Applications What are some more natural Divide and Conquer applications? Top down parser Mail delivery (divide by country/state/zip/etc.) 20-questions like binary search (only 1 sub-task) FFT (Fast Fourier Transform) hugely important/beneficial Polynomial multiplication is nlogn with DC FFT also transforms between time and frequency domains (speech, signal processing, etc.) Two closest points in a 2-d graph? - How and how long Brute force O(n 2 ), Divide and Conquer is O(nlogn) Divide and Conquer is also natural for parallelism CS Divide and Conquer Applications 52

53 Divide and Conquer Speed-up Take a problem whose basic (full n) algorithmic solution is complex in terms of n Consider sort and convex hull as examples Recursively divide (depth of log b n) to base cases which are now simple/ fast given small n (n=1 or some threshold) Speed-up happens when we can find short-cuts during partition/merge that can be taken because of the divide and conquer Don't just use same approach that could have been done at the top level Sort: fast merge with already sorted sub-lists Convex Hull: fast merge of ordered of sub-hulls (and dropping internal points while merging) Multiply: can use the "less multiplies trick" at each level of merge Quicksort: Partitioning is only O(n) at each level, but leads to final sorted list Binary Search/Selection: can discard half of the data at each level Master Theorem and recurrence relations tell us complexity CS Divide and Conquer Applications 53

54 Multiplication of Polynomials Key foundation to signal processing A(x) = 1 + 3x + 2x 2 Degree d=2 - highest power of x Coefficents a 0 = 1, a 1 = 3, a d=2 = 2 B(x) = 2 + 4x + x 2 A(x) B(x) = (1 + 3x + 2x 2 )(2 + 4x + x 2 ) = Polynomial of degree 2d Coefficients c 0, c 1,..., c 2d CS Divide and Conquer Applications 54

55 Multiplication of Polynomials More generally A(x) = a 0 + a 1 x + a 2 x a d x d B(x) = b 0 + b 1 x + b 2 x b d x d C(x) = A(x) B(x) = c 0 + c 1 x + c 2 x c 2d x 2d Just need to calculate coefficients c k for multiplication c k = a 0 b k + a 1 b k a k b 0 = Convolution Common operation in signal processing, etc. Complexity each c k = O(k) = O(d) 2d coefficients for total O(d 2 ) Can we do better? k a where a j,b m = 0 for j,m>d i b k i i= 0 CS Divide and Conquer Applications 55

56 Convolution k = y ( n) = h( n)* u( n) = u( k) h( n k) A(x) B(x) = (1 + 3x + 2x 2 )(2 + 4x + x 2 ) c 0 = 1 2 = 2 c 1 = = 10 c 2 = = 17 c 3 = = c 4 = 2 1 = CS Divide and Conquer Applications 56

57 1-D Convolution * = Smoothing (noise removal) CS Divide and Conquer Applications 57

58 2-D Convolution A 2-D signal (an Image) is convolved with a second Image (the filter, or convolution Kernel ). f(x,y) g(x,y) h(x,y)=f(x,y)*g(x,y) * = CS Divide and Conquer Applications 58

59 Faster Multiplication of Polynomials A degree-d polynomial is uniquely characterized by its values at any d +1 distinct points line, parabola, etc. Gives us two different ways to represent a polynomial A(x) = a 0 + a 1 x + a 2 x a d x d The coefficients a 0, a 1,..., a d The values A(x 1 ), A(x 2 ),..., A(x d ), for any distinct x i C(x) = A(x) B(x) can be represented by the values of 2d+1 distinct points, where each point C(z) = A(x) B(x) Thus in the Value Representation multiplication of polynomials takes 2d+1 multiplies and is O(d) Assumes we already have the values of A(z) and B(z) for 2d+1 distinct values CS Divide and Conquer Applications 59

60 Changing Representations We can get the value representation by evaluating the polynomial at distinct points using the original coefficient representation What is complexity? Each evaluation takes d multiplies. Thus to evaluate d points is O(d 2 ) Interpolation - We'll discuss in a minute CS Divide and Conquer Applications 60

61 An Algorithm Selection and Multiplication are O(d) What have we gained? But would if we could do evaluation and interpolation faster than O(d 2 ) CS Divide and Conquer Applications 61

62 Divide and Conquer Evaluation To put a polynomial of degree n-1 into the value representation we need to evaluate it at n distinct points If we choose our points cleverly we can use divide and conquer to get better than O(n 2 ) complexity Choose positive-negative pairs : ±x 0, ±x 1,..., ±x n/2-1 Computations for A(x i ) and A(-x i ) overlap a lot since even powers of x i coincide with those of -x i 3 + 4x + 9x 2 - x 3 +5x 4 + 2x 5 = (3 + 9x 2 +5x 4 ) + x(4 - x 2 + 2x 4 ) A(x i ) = A e (x i2 ) + x i A o (x i2 ) A(x) = 3 + 4x + 9x 2 - x 3 +5x 4 + 2x 5 = A e (x 2 ) + xa o (x 2 ) where A e (x) = (3 + 9x 2 +5x 4 ) A e (x 2 ) = (3 + 9x +5x 2 ) CS Divide and Conquer Applications 62

63 Worked out Example 3 + 4x + 9x 2 - x 3 +5x 4 + 2x 5 = (3 + 9x 2 +5x 4 ) + x(4 - x 2 + 2x 4 ) A(x i ) = A e (x i2 ) + x i A o (x i2 ) A(-x i ) = A e (x i2 ) - x i A o (x i2 ) A(x) = 3 + 4x + 9x 2 - x 3 +5x 4 + 2x 5 = A e (x 2 ) + xa o (x 2 ) where A e (x 2 ) = (3 + 9x +5x 2 ) and A o (x 2 ) = (4 - x + 2x 2 ) half the size and half the degree Try x = 2 and x = -2 and evaluate both ways 3 + 4x + 9x 2 - x 3 +5x 4 + 2x 5 = 3 + 4(2) + 9(4) - (8) +5(16) + 2(32) = (-2) + 9(4) - (-8) +5(16) + 2(-32) = 55 A(2) = A e (2 2 ) + 2A o (2 2 ) = 3 + 9(4) +5(16) + 2( (16)) = 183 A(-2) = A e (-2 2 ) - 2A o (-2 2 ) = 3 + 9(4) +5(16) - 2( (16)) = 55 CS Divide and Conquer Applications 63

64 Divide and Conquer Approach We divide the task into two sub-tasks each with half the size and with some linear time arithmetic required to combine. If we do this just once we cut the task in half but it is still O(n 2 ) If we continue the recursion we have t(n) = 2t(n/2) + O(n) which gives us the big improvement to O(nlogn) However, next level of positive-negative pairs? Negative squares? CS Divide and Conquer Applications 64

65 Complex n th Roots of Unity Assume final point in recursion is 1 Level above it must be its roots (1 and -1) This must continue up to initial problem size n The n complex solutions of z n = 1 Complex n th roots of unity CS Divide and Conquer Applications 65

66 8 th Roots of Unity " \$ # i % ' & 2 # = i 1 = ( 1) 2 = i 4 = % \$ i & ( ' 8 # = % \$ 2 i & ( ' 8 # % \$ 2 i & ( ' 2 # = i 1= ( 1) 2 = i 4 = % \$ 2 i & ( ' 8 # = % \$ i & ( ' 8 To make sure we get the positive-negative pairs we will use powers of 2 such that at each level k there are 2 k equally spaced points on the unit circle. These are not all the roots of unity But we just want points which have opposite points (differ by π) so that they are positive-negative pairs CS Divide and Conquer Applications 66

67 Complex Numbers Review Transformations between the complex plane and polar coordinates i = (1,π /4) =1(cosπ /4 + isinπ /4) =1eiπ / i = (1,5π /4) =1(cos5π /4 + isin5π /4) =1ei5π / 4 CS Divide and Conquer Applications 67

68 Complex Numbers Review (e iπ / 4 ) 2 = e iπ / 2 = i (e i5π / 4 ) 2 = e i5π / 2 = e iπ / 2 = i e icπ = e icπ +π (e icπ ) 2 = ( e icπ +π ) 2 (e iπ / 4 ) 4 = e iπ = 1 (e iπ / 4 ) 8 = e i2π =1 e π/4i is an 8th root of unity CS Divide and Conquer Applications 68

69 n th roots of unity are 1, ω, ω 2,..., ω n-1, where ω = e 2πi/n Note that the 3rd roots of unity are 1, e 2πi/3, and e 4πi/3, but they aren't plus-minus paired We'll use n values which are powers of 2 so that they stay plus-minus paired (even) through the entire recursion Polar (e 2πi/8 ) = e 0 (1,0) (e 2πi/8 ) 1 = e π/4 (1, π/4) (e 2πi/8 ) 2 = e π/2 (1, π/2) (e 2πi/8 ) 3 = e 3π/4 (1, 3π/4) (e 2πi/8 ) 4 = e π (1, π) (e 2πi/8 ) 5 = e 5π/4 (1, 5π/4) (e 2πi/8 ) 6 = e 3π/2 (1, 3π/2) (e 2πi/8 ) 7 = e 7π/4 CS Divide and Conquer Applications 69 (1, 7π/4) Cartesian 1 2 i -1 -i i i i i Value squared 1 i -1 -i 1 i -1 -i

70 Each step cuts work in half 2 subtasks per step Just linear set of adds and multiplies at each level t(n) = 2t(n/2) + O(n) Θ(nlogn)!! CS Divide and Conquer Applications 70

71 FFT Algorithm CS Divide and Conquer Applications 71

72 A(1) = = 3 A(-1) = = 5 A(i) = -i+2+3i+1 = 3+2i A(-i) = i+2-3i+1 = 3-2i ω = e 2πi/4 = i A(x) = x 3-2x 2 + 3x + 1 = (-2x 2 + 1) + x(x 2 + 3) ω = e 2πi/2 = -1 A(x) = -2x + 1 = (1) + x(-2) A(x) = (x + 3) = (3) + x(1) ω = e 2πi/1 = 1 A(x) = 1 A(x) = -2 A(x) = 3 A(x) = 1 A(1) =1 A(1) = -2 A(1) = 3 A(1) = 1 A(ω 0 ) = = -1 A(ω 0 ) = = 4 A(ω 1 ) = 1 + (-1)(-2) = 3 A(ω 1 ) = 3 + (-1)(1) = 2 A(ω 0 ) = -1 + (1)4 A(ω 1 ) = -1 + (-1)4 A(ω 2 ) = 3 + (i)2 A(ω 3 ) = 3 + (-i)2 = 3 = -5 = 3 + 2i = 3 2i CS Divide and Conquer Applications 72

73 A(1) = = 7 A(-1) = = 5 A(i) = 1+2i-4+3i+1 = -2+5i A(-i) = 1-2i-4-3i+1 = -2-5i ω = e 2πi/4 = i A(x) = x 4-2x 3 + 4x 2 + 3x + 1 = (x 4 + 4x 2 + 1) + x(-2x 2 + 3) ω = e 2πi/2 = -1 A(x) = x 2 +4x + 1 = (x 2 + 1) + x(4) A(x) = (-2x + 3) = (3) + x(-2) ω = e 2πi/1 = 1 A(x) = x + 1 A(x) = 4 A(x) = 3 A(x) = -2 A(1) =A(ω 0 )=2 A(1) = 4 A(1) = 3 A(1) = -2 A(ω 0 ) = 2 + (1)(4) = 6 A(ω 0 ) = 3 + (1)(-2) = 1 A(ω 1 ) = 2 + (-1)(4) = -2 A(ω 1 ) = 3 + (-1)(-2) = 5 A(ω 0 ) = 6+1=7 A(ω 1 ) = 6-1=5 A(ω 2 ) = -2+5i A(ω 3 ) = -2-5i CS Divide and Conquer Applications 73

74 Review Selection and Evaluation O(n) Now have Evaluation of O(nlogn) What about Interpolation? CS Divide and Conquer Applications 74

75 For any distinct set of points, evaluation is just the following Matrix Multiply This is a Vandermonde Matrix - given n distinct points it is always invertible Evaluation: A = M a Interpolation: a = M -1 A However, still O(n 2 ) for both operations for an arbitrary choice of points But, we can choose any points we want Fourier matrix is a Vandermonde matrix with n th roots of unity (with n a power of 2) Allows Evaluation in O(nlogn) due to its divide and conquer efficiencies Evaluation: Values = FFT(Coefficients, ω) ω -1 is an nth root of unity so interpolation can also be done with the FFT function: nlogn! Interpolation: Coefficients = 1/n FFT(Values, ω -1 ) CS Divide and Conquer Applications 75

76 Fast Fourier Transform Full polynomial multiplication is O(nlogn) with FFT FFT allows transformation between coefficient and value domain in an efficient manner Also transforms between time and frequency domains Other critical applications One of the most influential of all algorithms CS Divide and Conquer Applications 76

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