Agent Work: HW8: Greedy Algorithms
Claude Haiku 4.5 · COMP 182: Algorithmic Thinking
COMP 182: Algorithmic Thinking
Homework 8: Recurrences and Dynamic Programming
---
1. Modeling with Recurrences [50 pts]
Problem 1.1 [20 pts]
We would like to cover a rectangular area by two kinds of tiles: 1 x 1 and 1 x 2 tiles (the 1 x 2 tiles can be rotated). Give a recurrence and boundary conditions for the number of ways to cover the area with these two tiles if the area is:
1. 1 x n 2. 2 x n
Assume you have enough tiles of each kind. Show your work.
---
Problem 1.2 [10 pts]
Consider graph G = (V, E) where V = {1, 2, 3, 4, 5} and E = {{1,2}, {1,3}, {1,4}, {1,5}, {2,3}, {3,4}, {4,5}, {5,2}}.
Give a recurrence and the boundary conditions for the number of paths of length n that start at node 1 in this graph. Show your work.
---
Problem 1.3 [20 pts]
Consider the following set for n in N (natural numbers):
F = {f : {1, ..., n} -> {0, 1, 2} : for all 1 <= i <= n-1, f(i) + f(i+1) != 4}
(a) Give a recurrence and boundary conditions for |F|.
(b) Solve the recurrence and give an explicit formula for |F|.
Show your work in both parts.
---
2. Recurrences and Dynamic Programming [50 pts]
Problem 2.1 [25 pts]
We are given a color picture consisting of an m x n matrix A[1..m, 1..n] of pixels, where each pixel specifies a triple of red, green, and blue (RGB) intensities. Suppose that we wish to compress this picture slightly. Specifically, we wish to remove one pixel from each of the m rows, so that the whole picture becomes one pixel narrower. To avoid disturbing visual effects, however, we require that the pixels removed in two adjacent rows be in the same or adjacent columns; the pixels removed form a "seam" from the top row to the bottom row where successive pixels in the seam are adjacent vertically or diagonally.
(a) Show that the number of such possible seams grows at least exponentially in m, assuming that n > 1.
(b) Suppose now that along with each pixel A[i,j], we have calculated a real-valued disruption measure d[i,j], indicating how disruptive it would be to remove pixel A[i,j]. Intuitively, the lower a pixel's disruption measure, the more similar the pixel is to its neighbors. Suppose further that we define the disruption measure of a seam to be the sum of the disruption measures of its pixels. Give a polynomial-time dynamic programming algorithm to find a seam with the lowest disruption measure. First derive and show the recurrence and boundary conditions. How efficient is your algorithm?
---
Problem 2.2 [25 pts]
A certain string-processing language allows a programmer to break a string into two pieces. Because this operation copies the string, it costs n time units to break a string of n characters into two pieces. Suppose a programmer wants to break a string into many pieces. The order in which the breaks occur can affect the total amount of time used.
For example, suppose that the programmer wants to break a 20-character string after characters 2, 8, and 10 (numbering the characters in ascending order from the left-hand end, starting from 1).
- If she programs the breaks to occur in left-to-right order, then the first break costs 20 time units, the second break costs 18 time units (breaking the string from characters 3 to 20 at character 8), and the third break costs 12 time units, totaling 50 time units.
- If she programs the breaks to occur in right-to-left order, however, then the first break costs 20 time units, the second break costs 10 time units, and the third break costs 8 time units, totaling 38 time units.
- In yet another order, she could break first at 8 (costing 20), then break the left piece at 2 (costing 8), and finally the right piece at 10 (costing 12), for a total cost of 40.
Give a polynomial-time dynamic programming algorithm that, given the numbers of characters after which to break, determines a least-cost way to sequence those breaks. More formally, given a string S with n characters and an array L[1..m] containing the break points, compute the lowest cost for a sequence of breaks, along with a sequence of breaks that achieves this cost.
First derive and show the recurrence and boundary conditions. How efficient is your algorithm?
# COMP182 Homework 8
## Student Responses
<!-- Write your solutions below. Add sections for each problem as needed. -->
<!-- Use proper mathematical notation where applicable. -->
[Your solutions here]
COMP182 Homework 8 Solutions
Problem 1.1: Tiling Problems
Part 1.1a: Tiling a 1 × n Area
For a 1×n rectangular strip with 1×1 tiles and 1×2 tiles (which can be rotated):
- In a 1×n strip, a 1×2 tile must be placed horizontally (2 wide, 1 tall) to fit
- The available tiles are: 1×1 and 2×1 (rotated 1×2)
Recurrence: Let f(n) = number of ways to cover a 1×n strip.
We can fill the rightmost end of the strip in two ways: 1. Place a 1×1 tile at position n, leaving a 1×(n-1) strip: f(n-1) ways 2. Place a 2×1 tile at positions (n-1, n), leaving a 1×(n-2) strip: f(n-2) ways
Recurrence Relation:
f(n) = f(n-1) + f(n-2) for n ≥ 2Boundary Conditions:
f(0) = 1 (one way to tile empty strip: use no tiles)
f(1) = 1 (one way to tile 1×1 strip: use one 1×1 tile)This is the Fibonacci recurrence.
---
Part 1.1b: Tiling a 2 × n Area
For a 2×n rectangular grid with 1×1, 1×2 (1 wide, 2 tall), and 2×1 (2 wide, 1 tall) tiles:
To derive the recurrence, I'll track states at the boundary between filled and unfilled columns.
Let:
- f(n, 0) = ways to tile n columns such that no tiles extend beyond column n
- f(n, 1) = ways to tile n columns with top row extending beyond (bottom complete)
- f(n, 2) = ways to tile n columns with bottom row extending beyond (top complete)
- f(n, 3) = ways to tile n columns with both rows extending beyond
Transitions from a fully complete boundary (state 0):
- Place two 1×1 tiles in column n+1: stays in state 0
- Place one 1×2 tile in column n+1: stays in state 0
- Place 2×1 tile in top row extending to n+1: goes to state 1
- Place 2×1 tile in bottom row extending to n+1: goes to state 2
- Place 2×1 in both rows: goes to state 3
From states 1, 2, 3: must complete the remaining cells with 1×1 tiles, returning to state 0.
This gives:
- g(n) = f(n, 0) satisfies: g(n) = 2·g(n-1) + 3·g(n-2)
Recurrence Relation:
g(n) = 2·g(n-1) + 3·g(n-2) for n ≥ 2Boundary Conditions:
g(0) = 1 (one way to tile empty grid)
g(1) = 2 (two 1×1 tiles stacked OR one 1×2 tile)Verification:
- g(2) = 2·2 + 3·1 = 7 ✓ (matches enumeration of 7 distinct tilings of 2×2)
---
Problem 1.2: Paths in a Graph
Given: G = (V, E) with V = {1,2,3,4,5} and E = {{1,2}, {1,3}, {1,4}, {1,5}, {2,3}, {3,4}, {4,5}, {5,2}}
Assuming undirected edges (standard notation {u,v}), the adjacency is:
- Node 1: neighbors {2, 3, 4, 5}
- Node 2: neighbors {1, 3, 5}
- Node 3: neighbors {1, 2, 4}
- Node 4: neighbors {1, 3, 5}
- Node 5: neighbors {1, 2, 4}
Recurrence Relations: Let p_i(n) = number of paths of length n starting at node 1 and ending at node i.
For n ≥ 1:
p_1(n) = p_2(n-1) + p_3(n-1) + p_4(n-1) + p_5(n-1)
p_2(n) = p_1(n-1) + p_3(n-1) + p_5(n-1)
p_3(n) = p_1(n-1) + p_2(n-1) + p_4(n-1)
p_4(n) = p_1(n-1) + p_3(n-1) + p_5(n-1)
p_5(n) = p_1(n-1) + p_2(n-1) + p_4(n-1)Boundary Conditions:
p_1(0) = 1 (path of length 0 is the starting node 1)
p_i(0) = 0 for i ∈ {2, 3, 4, 5}The total number of paths of length n starting at node 1 is: p(n) = Σ p_i(n) for i ∈ {1,2,3,4,5}
---
Problem 1.3: Functions with Constraints
Given: F = {f : {1,...,n} → {0,1,2} : for all 1 ≤ i ≤ n-1, f(i) + f(i+1) ≠ 4}
Part (a): Recurrence and Boundary Conditions
The constraint f(i) + f(i+1) ≠ 4 rules out only the case where both consecutive values are 2.
Define:
- a(n) = |F| (number of valid functions on {1,...,n})
- c(n) = number of valid functions on {1,...,n} with f(n) = 2
Deriving the recurrence:
For f ∈ F defined on {1,...,n}:
- If f(n) = 0 or 1: f(n-1) can be any value in {0,1,2}. These contribute a(n-1) functions each.
- If f(n) = 2: then f(n-1) + f(n) ≠ 4 requires f(n-1) ≠ 2. These contribute a(n-1) - c(n-1) functions.
Thus:
a(n) = a(n-1) + a(n-1) + (a(n-1) - c(n-1)) = 3·a(n-1) - c(n-1)And:
c(n) = a(n-1) - c(n-1) (functions ending in 2 must have f(n-1) ≠ 2)From these relations:
- a(n) - c(n) = 2·a(n-1)
- a(n) = 2·a(n-1) + c(n)
- c(n) = 2·a(n-2) (can be derived by substitution)
Therefore:
a(n) = 2·a(n-1) + 2·a(n-2) for n ≥ 2Boundary Conditions:
a(0) = 1 (one way to assign values on empty domain: the empty function)
a(1) = 3 (three functions: f(1)∈{0,1,2}, no constraint)Verification:
- a(2) = 2·3 + 2·1 = 8 ✓
- a(3) = 2·8 + 2·3 = 22 ✓
---
Part (b): Explicit Formula
The recurrence a(n) = 2·a(n-1) + 2·a(n-2) has characteristic equation:
x² - 2x - 2 = 0Using the quadratic formula:
x = (2 ± √(4 + 8))/2 = (2 ± 2√3)/2 = 1 ± √3The roots are: r₁ = 1 + √3 and r₂ = 1 - √3
The general solution is:
a(n) = A·(1 + √3)ⁿ + B·(1 - √3)ⁿUsing boundary conditions a(0) = 1 and a(1) = 3:
- a(0) = A + B = 1
- a(1) = A(1 + √3) + B(1 - √3) = 3
Solving:
- From the first: B = 1 - A
- Substituting: A(1 + √3) + (1-A)(1 - √3) = 3
- A(2√3) + (1 - √3) = 3
- A = (2 + √3)/(2√3) = (2√3 + 3)/6
And B = (3 - 2√3)/6
Explicit Formula:
a(n) = [(2√3 + 3)/6]·(1 + √3)ⁿ + [(3 - 2√3)/6]·(1 - √3)ⁿOr equivalently:
a(n) = (1/6)·[(2√3 + 3)·(1 + √3)ⁿ + (3 - 2√3)·(1 - √3)ⁿ]---
Problem 2.1: Seam Removal for Image Compression
Part (a): Exponential Growth of Seams
A seam is a sequence of pixels (one per row) from top to bottom where consecutive pixels are vertically or diagonally adjacent (in the same or adjacent columns).
Number of possible seams:
For an m×n grid with m rows and n columns:
- Row 1: pixel can be in any of n columns
- Row 2: pixel can be in one of 3 columns (same column or ±1 columns from previous)
- Rows 3 to m: each can be in one of 3 columns (relative to previous row)
In the worst case (avoiding boundary effects by assuming n is large):
- Number of seams ≥ n · 3^(m-1)
Since 3^(m-1) grows exponentially in m and n > 1, the number of seams grows at least exponentially in m.
For large m, the growth is approximately O(3^m), which is exponential.
---
Part (b): Dynamic Programming Algorithm for Minimum-Disruption Seam
Problem: Find a seam with minimum total disruption cost.
Recurrence:
Let DP[i][j] = minimum disruption cost of a seam from row 1 to row i, ending at column j.
DP[1][j] = d[1][j] for all j ∈ {1,...,n}For i > 1:
DP[i][j] = d[i][j] + min(DP[i-1][j-1], DP[i-1][j], DP[i-1][j+1])(where boundary conditions apply: DP[i-1][j-1] = ∞ if j-1 < 1, etc.)
Boundary Conditions:
DP[1][j] = d[1][j] for j ∈ {1,...,n}Algorithm:
1. Initialize: DP[1][j] = d[1][j] for all j
2. For i = 2 to m:
For j = 1 to n:
DP[i][j] = d[i][j] + min of (DP[i-1][j-1], DP[i-1][j], DP[i-1][j+1])
(handling boundaries appropriately)
3. Answer = min(DP[m][j]) for all j ∈ {1,...,n}
4. Backtrack to reconstruct the seamEfficiency:
- Time complexity: O(m·n) — iterate through m rows and n columns, each cell examined once
- Space complexity: O(m·n) for the DP table, or O(n) if using rolling arrays since only previous row is needed
- Overall: Polynomial time (linear in input size)
This is a classic dynamic programming problem similar to the matrix chain multiplication or longest common subsequence problems.
---
Problem 2.2: Optimal String Breaking Sequence
Problem Setup
Given a string of n characters and an array L[1..m] of m break points (positions after which breaks occur).
Objective: Find the sequence of breaks that minimizes total cost, where breaking a string of length ℓ costs ℓ time units.
Recurrence and Algorithm
Insight: The problem is a partition optimization problem. After breaks are made, we have m+1 substrings. The order of breaks affects which substring gets broken when.
Let the positions be extended as p₀ = 0 and p_{m+1} = n (full string), with L[1..m] = p₁, p₂, ..., p_m in sorted order.
Define: DP[i][j] = minimum cost to completely partition substring from position p_i to p_j (which has length p_j - p_i).
This represents the cost of performing all internal breaks to further split the substring into parts matching the required break points within [p_i, p_j].
Recurrence:
If no further breaks are needed within (p_i, p_j):
DP[i][i+1] = 0 (substring with no internal breaks requires no breaks)For segments requiring internal breaks:
DP[i][j] = (p_j - p_i) + min{ DP[i][k] + DP[k][j] : i < k < j }The cost (p_j - p_i) is incurred by the first break that splits the segment into two parts at some internal position k.
Boundary Conditions:
DP[i][i+1] = 0 for all i (no breaks needed for adjacent boundaries)Base Case for Computation:
Compute DP values in increasing order of segment length (j - i):
1. Length 1: DP[i][i+1] = 0 for all i 2. Length 2: DP[i][i+2] = (p_{i+2} - p_i) + 0 = p_{i+2} - p_i (one break needed) 3. Length k: DP[i][i+k] = (p_{i+k} - p_i) + min{ DP[i][i+t] + DP[i+t][i+k] : 1 ≤ t < k }
Algorithm:
1. Sort break points: L[1..m]
2. Extend: p[0]=0, p[1..m]=L[1..m], p[m+1]=n
3. Initialize DP[i][i+1] = 0 for all i
4. For len = 2 to m+1:
For i = 0 to m+1-len:
j = i + len
DP[i][j] = ∞
cost_of_break = p[j] - p[i]
For k = i+1 to j-1:
DP[i][j] = min(DP[i][j], cost_of_break + DP[i][k] + DP[k][j])
5. Answer = DP[0][m+1]
6. Reconstruct by tracking which k achieved the minimum at each stepReconstruction: To output the sequence of breaks, maintain parent pointers tracking which k value was chosen at each DP[i][j], then recursively reconstruct the left and right subproblems.
Efficiency
- Time complexity: O(m³) — three nested loops over break positions
- Space complexity: O(m²) — DP table of size (m+2)×(m+2)
- Overall: Polynomial time
This is a classic interval DP problem, related to matrix chain multiplication and optimal binary search trees.
COMP 182 Homework 8 — Grading Report
---
Problem 1.1, Part (1): 1 × n Tiling [6/6]
Recurrence (3/3): The student correctly derives f(n) = f(n−1) + f(n−2) by considering the rightmost tile placement (1×1 or 1×2 horizontal). This matches the reference.
Boundary Conditions (3/3): The student gives f(0) = 1 and f(1) = 1. While the reference uses a₁ = 1 and a₂ = 2, the student's formulation is equivalent — f(2) = f(1) + f(0) = 2, matching a₂ = 2. Using f(0) = 1 (empty strip, one tiling using no tiles) is a standard and valid choice.
---
Problem 1.1, Part (2): 2 × n Tiling [6/14]
Identifying auxiliary structure (2/4): The student correctly identifies the need for tracking state at column boundaries and defines four states (0, 1, 2, 3) based on which rows extend. This is a legitimate approach analogous to auxiliary sequences. However, the state transition analysis for non-zero states is incomplete.
Case analysis (2/4): The transitions *from state 0* are correctly enumerated (5 cases: two 1×1's, one vertical tile, horizontal top+1×1 bottom, 1×1 top+horizontal bottom, horizontal in both). However, the student's claim that "From states 1, 2, 3: must complete the remaining cells with 1×1 tiles, returning to state 0" is incorrect. From state 1 (top extends, bottom empty), one can also place a horizontal 1×2 tile in the bottom row, transitioning to state 2 — not just a 1×1 tile returning to state 0. Similarly, from state 2 one can transition to state 1. These missed transitions are a significant error.
Combined recurrence (0/3): The resulting recurrence g(n) = 2g(n−1) + 3g(n−2) is incorrect. Verification: g(3) = 2·7 + 3·2 = 20, but the correct value is 22 (confirmed by transfer matrix computation). The correct recurrence is a_n = 3a_{n−1} + a_{n−2} − a_{n−3}.
Boundary conditions (2/3): The individual values g(0) = 1 and g(1) = 2 are correct (there are indeed 2 tilings of a 2×1 grid and 1 tiling of an empty grid). However, they correspond to the wrong recurrence.
---
Problem 1.2: Graph Paths [6/10]
Symmetry recognition (1/3): The student does not explicitly recognize the symmetry of the graph (nodes 2,3,4,5 form a 4-cycle with rotational symmetry, so all have equivalent path counts). The recurrences implicitly show p₂ = p₄ and p₃ = p₅, but this observation is never stated or exploited.
System of recurrences (3/3): The full system of five recurrences is correct. The adjacency lists are accurately identified, and the recurrence p_i(n) = Σ p_j(n−1) over neighbors j of i is properly applied.
Combined recurrence (0/2): The student does not derive a single recurrence for the total number of paths. The reference derives a_n = 2a_{n−1} + 4a_{n−2} by exploiting symmetry and eliminating the auxiliary variable. This simplification is missing.
Boundary conditions (2/2): p₁(0) = 1 and p_i(0) = 0 for i ∈ {2,3,4,5} are correct. Verified: total paths of length 1 = 4 and length 2 = 12, matching the reference.
---
Problem 1.3, Part (a): Recurrence for |F| [14/14]
Problem interpretation (5/5): The student correctly identifies that f(i) + f(i+1) ≠ 4 only excludes consecutive 2's (since 2+2 = 4 is the only way to reach 4 in {0,1,2}).
Recurrence with justification (5/5): The derivation is thorough and correct. The student introduces an auxiliary count c(n) for functions ending in 2, derives the system a(n) = 3a(n−1) − c(n−1) and c(n) = a(n−1) − c(n−1), then elegantly eliminates c to obtain a(n) = 2a(n−1) + 2a(n−2). This matches the reference exactly.
Boundary conditions (4/4): a(0) = 1 and a(1) = 3 are correct. The student verifies: a(2) = 8 (= 9 − 1 pair of consecutive 2's ✓) and a(3) = 22 ✓.
---
Problem 1.3, Part (b): Explicit Formula [6/6]
Characteristic equation (2/2): x² − 2x − 2 = 0 ✓
Characteristic roots (2/2): r₁ = 1 + √3, r₂ = 1 − √3 ✓
Explicit formula (2/2): The student correctly solves for coefficients:
- A = (2√3 + 3)/6, B = (3 − 2√3)/6
- a(n) = [(2√3+3)/6]·(1+√3)ⁿ + [(3−2√3)/6]·(1−√3)ⁿ
Verified: a(0) = 6/6 = 1 ✓, a(1) = 18/6 = 3 ✓. The formula is correct.
---
Problem 2.1, Part (a): Exponential Seam Growth [4/5]
Argument (4/5): The student correctly argues that each row (beyond the first) offers up to 3 choices for the next pixel, yielding at least n · 3^(m−1) seams. The conclusion that growth is exponential in m is correct. However, the argument includes the qualifier "avoiding boundary effects by assuming n is large," which doesn't rigorously handle the n = 2 case (where the problem states n > 1). A cleaner argument (as in the reference) would note that even boundary columns give at least 2 choices, so the number of seams is at least 2^m for any n > 1. Minor deduction for this gap in rigor.
---
Problem 2.1, Part (b): Seam Carving DP [20/20]
Subproblem definition (5/5): DP[i][j] = minimum disruption of a seam from row 1 to row i ending at column j. Clear and correct.
Recurrence (5/5): DP[i][j] = d[i][j] + min(DP[i−1][j−1], DP[i−1][j], DP[i−1][j+1]) with proper boundary handling (∞ for out-of-bounds). Correctly handles left edge, right edge, and interior cases.
Boundary conditions (3/3): DP[1][j] = d[1][j] for all j ∈ {1,...,n}. ✓
Pseudo-code (4/4): Clear algorithm: initialize first row, fill row-by-row, find minimum in last row, backtrack. Correct order of computation.
Time complexity (3/3): O(m·n) time, O(m·n) or O(n) space. Correctly identified as polynomial. ✓
---
Problem 2.2: Optimal String Breaking [25/25]
Definition and extended L (5/5): Correctly extends L with p₀ = 0 and p_{m+1} = n. DP[i][j] defined as minimum cost to partition substring from p_i to p_j. Matches the reference.
Recurrence (5/5): DP[i][j] = (p_j − p_i) + min{DP[i][k] + DP[k][j] : i < k < j}. The term (p_j − p_i) correctly accounts for the cost of the first break, and the recursive subproblems are correctly formulated. Clear justification provided.
Boundary conditions (3/3): DP[i][i+1] = 0 for all i (no internal break points → no breaks needed). ✓
Pseudo-code (5/5): Correctly computes in increasing order of segment length (j − i), iterating over all valid (i, j) pairs and all possible break points k. The algorithm structure mirrors optimal BST / matrix chain multiplication.
Time complexity (4/4): O(m³) time (three nested loops), O(m²) space. ✓
Traceback (3/3): Explicitly mentions maintaining parent pointers and recursive reconstruction of the break sequence. ✓
---
Summary
| Problem | Earned | Max |
|---|---|---|
| 1.1 (1×n) | 6 | 6 |
| 1.1 (2×n) | 6 | 14 |
| 1.2 | 6 | 10 |
| 1.3 (a) | 14 | 14 |
| 1.3 (b) | 6 | 6 |
| 2.1 (a) | 4 | 5 |
| 2.1 (b) | 20 | 20 |
| 2.2 | 25 | 25 |
| Total | 87 | 100 |
Overall Comments: Strong submission with excellent work on Problems 1.3, 2.1(b), and 2.2 — all earning full or near-full marks. The dynamic programming solutions are particularly well-presented with clear definitions, recurrences, pseudocode, and complexity analysis. The main point losses come from Problem 1.1(2×n), where the state-based tiling approach was a sound idea but the transitions from non-zero states were oversimplified (missing the option to place horizontal tiles when one row is partially filled), leading to an incorrect recurrence. In Problem 1.2, the system of recurrences is correct but would benefit from exploiting the graph's symmetry to derive a single closed-form recurrence.
Total: 87/100
diff --git a/writeup.md b/writeup.md
index 518f933..d361d64 100644
--- a/writeup.md
+++ b/writeup.md
@@ -1,9 +1,336 @@
-# COMP182 Homework 8
+# COMP182 Homework 8 Solutions
-## Student Responses
+## Problem 1.1: Tiling Problems
-<!-- Write your solutions below. Add sections for each problem as needed. -->
-<!-- Use proper mathematical notation where applicable. -->
+### Part 1.1a: Tiling a 1 × n Area
-[Your solutions here]
+For a 1×n rectangular strip with 1×1 tiles and 1×2 tiles (which can be rotated):
+- In a 1×n strip, a 1×2 tile must be placed horizontally (2 wide, 1 tall) to fit
+- The available tiles are: 1×1 and 2×1 (rotated 1×2)
+**Recurrence:**
+Let f(n) = number of ways to cover a 1×n strip.
+
+We can fill the rightmost end of the strip in two ways:
+1. Place a 1×1 tile at position n, leaving a 1×(n-1) strip: f(n-1) ways
+2. Place a 2×1 tile at positions (n-1, n), leaving a 1×(n-2) strip: f(n-2) ways
+
+**Recurrence Relation:**
+```
+f(n) = f(n-1) + f(n-2) for n ≥ 2
+```
+
+**Boundary Conditions:**
+```
+f(0) = 1 (one way to tile empty strip: use no tiles)
+f(1) = 1 (one way to tile 1×1 strip: use one 1×1 tile)
+```
+
+This is the Fibonacci recurrence.
+
+---
+
+### Part 1.1b: Tiling a 2 × n Area
+
+For a 2×n rectangular grid with 1×1, 1×2 (1 wide, 2 tall), and 2×1 (2 wide, 1 tall) tiles:
+
+To derive the recurrence, I'll track states at the boundary between filled and unfilled columns.
+
+Let:
+- f(n, 0) = ways to tile n columns such that no tiles extend beyond column n
+- f(n, 1) = ways to tile n columns with top row extending beyond (bottom complete)
+- f(n, 2) = ways to tile n columns with bottom row extending beyond (top complete)
+- f(n, 3) = ways to tile n columns with both rows extending beyond
+
+Transitions from a fully complete boundary (state 0):
+- Place two 1×1 tiles in column n+1: stays in state 0
+- Place one 1×2 tile in column n+1: stays in state 0
+- Place 2×1 tile in top row extending to n+1: goes to state 1
+- Place 2×1 tile in bottom row extending to n+1: goes to state 2
+- Place 2×1 in both rows: goes to state 3
+
+From states 1, 2, 3: must complete the remaining cells with 1×1 tiles, returning to state 0.
+
+This gives:
+- g(n) = f(n, 0) satisfies: g(n) = 2·g(n-1) + 3·g(n-2)
+
+**Recurrence Relation:**
+```
+g(n) = 2·g(n-1) + 3·g(n-2) for n ≥ 2
+```
+
+**Boundary Conditions:**
+```
+g(0) = 1 (one way to tile empty grid)
+g(1) = 2 (two 1×1 tiles stacked OR one 1×2 tile)
+```
+
+Verification:
+- g(2) = 2·2 + 3·1 = 7 ✓ (matches enumeration of 7 distinct tilings of 2×2)
+
+---
+
+## Problem 1.2: Paths in a Graph
+
+Given: G = (V, E) with V = {1,2,3,4,5} and E = {{1,2}, {1,3}, {1,4}, {1,5}, {2,3}, {3,4}, {4,5}, {5,2}}
+
+Assuming undirected edges (standard notation {u,v}), the adjacency is:
+- Node 1: neighbors {2, 3, 4, 5}
+- Node 2: neighbors {1, 3, 5}
+- Node 3: neighbors {1, 2, 4}
+- Node 4: neighbors {1, 3, 5}
+- Node 5: neighbors {1, 2, 4}
+
+**Recurrence Relations:**
+Let p_i(n) = number of paths of length n starting at node 1 and ending at node i.
+
+For n ≥ 1:
+```
+p_1(n) = p_2(n-1) + p_3(n-1) + p_4(n-1) + p_5(n-1)
+p_2(n) = p_1(n-1) + p_3(n-1) + p_5(n-1)
+p_3(n) = p_1(n-1) + p_2(n-1) + p_4(n-1)
+p_4(n) = p_1(n-1) + p_3(n-1) + p_5(n-1)
+p_5(n) = p_1(n-1) + p_2(n-1) + p_4(n-1)
+```
+
+**Boundary Conditions:**
+```
+p_1(0) = 1 (path of length 0 is the starting node 1)
+p_i(0) = 0 for i ∈ {2, 3, 4, 5}
+```
+
+The total number of paths of length n starting at node 1 is: p(n) = Σ p_i(n) for i ∈ {1,2,3,4,5}
+
+---
+
+## Problem 1.3: Functions with Constraints
+
+Given: F = {f : {1,...,n} → {0,1,2} : for all 1 ≤ i ≤ n-1, f(i) + f(i+1) ≠ 4}
+
+### Part (a): Recurrence and Boundary Conditions
+
+The constraint f(i) + f(i+1) ≠ 4 rules out only the case where both consecutive values are 2.
+
+Define:
+- a(n) = |F| (number of valid functions on {1,...,n})
+- c(n) = number of valid functions on {1,...,n} with f(n) = 2
+
+**Deriving the recurrence:**
+
+For f ∈ F defined on {1,...,n}:
+- If f(n) = 0 or 1: f(n-1) can be any value in {0,1,2}. These contribute a(n-1) functions each.
+- If f(n) = 2: then f(n-1) + f(n) ≠ 4 requires f(n-1) ≠ 2. These contribute a(n-1) - c(n-1) functions.
+
+Thus:
+```
+a(n) = a(n-1) + a(n-1) + (a(n-1) - c(n-1)) = 3·a(n-1) - c(n-1)
+```
+
+And:
+```
+c(n) = a(n-1) - c(n-1) (functions ending in 2 must have f(n-1) ≠ 2)
+```
+
+From these relations:
+- a(n) - c(n) = 2·a(n-1)
+- a(n) = 2·a(n-1) + c(n)
+- c(n) = 2·a(n-2) (can be derived by substitution)
+
+Therefore:
+```
+a(n) = 2·a(n-1) + 2·a(n-2) for n ≥ 2
+```
+
+**Boundary Conditions:**
+```
+a(0) = 1 (one way to assign values on empty domain: the empty function)
+a(1) = 3 (three functions: f(1)∈{0,1,2}, no constraint)
+```
+
+Verification:
+- a(2) = 2·3 + 2·1 = 8 ✓
+- a(3) = 2·8 + 2·3 = 22 ✓
+
+---
+
+### Part (b): Explicit Formula
+
+The recurrence a(n) = 2·a(n-1) + 2·a(n-2) has characteristic equation:
+```
+x² - 2x - 2 = 0
+```
+
+Using the quadratic formula:
+```
+x = (2 ± √(4 + 8))/2 = (2 ± 2√3)/2 = 1 ± √3
+```
+
+The roots are: r₁ = 1 + √3 and r₂ = 1 - √3
+
+The general solution is:
+```
+a(n) = A·(1 + √3)ⁿ + B·(1 - √3)ⁿ
+```
+
+Using boundary conditions a(0) = 1 and a(1) = 3:
+- a(0) = A + B = 1
+- a(1) = A(1 + √3) + B(1 - √3) = 3
+
+Solving:
+- From the first: B = 1 - A
+- Substituting: A(1 + √3) + (1-A)(1 - √3) = 3
+- A(2√3) + (1 - √3) = 3
+- A = (2 + √3)/(2√3) = (2√3 + 3)/6
+
+And B = (3 - 2√3)/6
+
+**Explicit Formula:**
+```
+a(n) = [(2√3 + 3)/6]·(1 + √3)ⁿ + [(3 - 2√3)/6]·(1 - √3)ⁿ
+```
+
+Or equivalently:
+```
+a(n) = (1/6)·[(2√3 + 3)·(1 + √3)ⁿ + (3 - 2√3)·(1 - √3)ⁿ]
+```
+
+---
+
+## Problem 2.1: Seam Removal for Image Compression
+
+### Part (a): Exponential Growth of Seams
+
+A seam is a sequence of pixels (one per row) from top to bottom where consecutive pixels are vertically or diagonally adjacent (in the same or adjacent columns).
+
+**Number of possible seams:**
+
+For an m×n grid with m rows and n columns:
+- Row 1: pixel can be in any of n columns
+- Row 2: pixel can be in one of 3 columns (same column or ±1 columns from previous)
+- Rows 3 to m: each can be in one of 3 columns (relative to previous row)
+
+In the worst case (avoiding boundary effects by assuming n is large):
+- Number of seams ≥ n · 3^(m-1)
+
+Since 3^(m-1) grows exponentially in m and n > 1, the number of seams grows **at least exponentially in m**.
+
+For large m, the growth is approximately O(3^m), which is exponential.
+
+---
+
+### Part (b): Dynamic Programming Algorithm for Minimum-Disruption Seam
+
+**Problem:** Find a seam with minimum total disruption cost.
+
+**Recurrence:**
+
+Let DP[i][j] = minimum disruption cost of a seam from row 1 to row i, ending at column j.
+
+```
+DP[1][j] = d[1][j] for all j ∈ {1,...,n}
+```
+
+For i > 1:
+```
+DP[i][j] = d[i][j] + min(DP[i-1][j-1], DP[i-1][j], DP[i-1][j+1])
+```
+(where boundary conditions apply: DP[i-1][j-1] = ∞ if j-1 < 1, etc.)
+
+**Boundary Conditions:**
+```
+DP[1][j] = d[1][j] for j ∈ {1,...,n}
+```
+
+**Algorithm:**
+
+```
+1. Initialize: DP[1][j] = d[1][j] for all j
+2. For i = 2 to m:
+ For j = 1 to n:
+ DP[i][j] = d[i][j] + min of (DP[i-1][j-1], DP[i-1][j], DP[i-1][j+1])
+ (handling boundaries appropriately)
+3. Answer = min(DP[m][j]) for all j ∈ {1,...,n}
+4. Backtrack to reconstruct the seam
+```
+
+**Efficiency:**
+
+- Time complexity: **O(m·n)** — iterate through m rows and n columns, each cell examined once
+- Space complexity: **O(m·n)** for the DP table, or **O(n)** if using rolling arrays since only previous row is needed
+- Overall: **Polynomial time** (linear in input size)
+
+This is a classic dynamic programming problem similar to the matrix chain multiplication or longest common subsequence problems.
+
+---
+
+## Problem 2.2: Optimal String Breaking Sequence
+
+### Problem Setup
+
+Given a string of n characters and an array L[1..m] of m break points (positions after which breaks occur).
+
+Objective: Find the sequence of breaks that minimizes total cost, where breaking a string of length ℓ costs ℓ time units.
+
+### Recurrence and Algorithm
+
+**Insight:** The problem is a partition optimization problem. After breaks are made, we have m+1 substrings. The order of breaks affects which substring gets broken when.
+
+Let the positions be extended as p₀ = 0 and p_{m+1} = n (full string), with L[1..m] = p₁, p₂, ..., p_m in sorted order.
+
+**Define:** DP[i][j] = minimum cost to completely partition substring from position p_i to p_j (which has length p_j - p_i).
+
+This represents the cost of performing all internal breaks to further split the substring into parts matching the required break points within [p_i, p_j].
+
+**Recurrence:**
+
+If no further breaks are needed within (p_i, p_j):
+```
+DP[i][i+1] = 0 (substring with no internal breaks requires no breaks)
+```
+
+For segments requiring internal breaks:
+```
+DP[i][j] = (p_j - p_i) + min{ DP[i][k] + DP[k][j] : i < k < j }
+```
+
+The cost (p_j - p_i) is incurred by the first break that splits the segment into two parts at some internal position k.
+
+**Boundary Conditions:**
+```
+DP[i][i+1] = 0 for all i (no breaks needed for adjacent boundaries)
+```
+
+**Base Case for Computation:**
+
+Compute DP values in increasing order of segment length (j - i):
+
+1. Length 1: DP[i][i+1] = 0 for all i
+2. Length 2: DP[i][i+2] = (p_{i+2} - p_i) + 0 = p_{i+2} - p_i (one break needed)
+3. Length k: DP[i][i+k] = (p_{i+k} - p_i) + min{ DP[i][i+t] + DP[i+t][i+k] : 1 ≤ t < k }
+
+**Algorithm:**
+
+```
+1. Sort break points: L[1..m]
+2. Extend: p[0]=0, p[1..m]=L[1..m], p[m+1]=n
+3. Initialize DP[i][i+1] = 0 for all i
+4. For len = 2 to m+1:
+ For i = 0 to m+1-len:
+ j = i + len
+ DP[i][j] = ∞
+ cost_of_break = p[j] - p[i]
+ For k = i+1 to j-1:
+ DP[i][j] = min(DP[i][j], cost_of_break + DP[i][k] + DP[k][j])
+5. Answer = DP[0][m+1]
+6. Reconstruct by tracking which k achieved the minimum at each step
+```
+
+**Reconstruction:** To output the sequence of breaks, maintain parent pointers tracking which k value was chosen at each DP[i][j], then recursively reconstruct the left and right subproblems.
+
+### Efficiency
+
+- **Time complexity:** O(m³) — three nested loops over break positions
+- **Space complexity:** O(m²) — DP table of size (m+2)×(m+2)
+- **Overall:** **Polynomial time**
+
+This is a classic interval DP problem, related to matrix chain multiplication and optimal binary search trees.
Sub-Model Usage
| Model | Input | Output | Cache Read | Cost |
|---|---|---|---|---|
| claude-haiku-4-5-20251001 | 23 | 37.6K | 77.3K | $0.26 |
BSCS Bench