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# Dynamic Programming

Delve into the fascinating world of dynamic programming, a core component of mathematical problem-solving. The subsequent text takes a comprehensive look at the complex definition, fundamental principles, and general methods underpinning this topic. It will guide you through practical examples, drawing on the differences between dynamic and linear programming. Additionally, you will embark on specific strategies such as minimax and maximin while diving deeper into the usage of dynamic programming in mathematical quandaries. This educational journey will broaden your knowledge, offering added insights into this intricate mathematical method.

## Understanding Dynamic Programming

Dynamic Programming is a fundamental concept in computer science, especially for solving optimization problems. Combining mathematical strategies and coding intelligence, it is a method that helps simplify complex problems, making them more manageable.

### Dynamic Programming: A Comprehensive Definition

Dynamic Programming is a mathematical and programming method used in computation to solve problems by breaking them down into simpler subproblems. It avoids re-computing solutions, optimising the code's effectiveness by constructing a table of results for subproblems and using these stored results when needed.

### The Basic Principle Behind Dynamic Programming

Dynamic Programming works on the principle of optimality. It utilises the idea of breaking down a problem into smaller subproblems, solving each one individually and utilising those results to solve the larger, overarching problem. The stored solutions then support the solving of dependant subproblems, preventing repeated calculations and saving computational resources.

• Problem Decomposition: The original problem is split into smaller subproblems
• Solving Subproblems: Each subproblem is solved independently
• Storage of Solutions: The solutions to the subproblems are stored for later use
• Reuse of Solutions: Saved solutions are re-used as-needed for related subproblems

#### General Method of Dynamic Programming

The general method of Dynamic Programming follows four key steps: Characterisation, Recursion, Bottom-Up Computation, and Construction of an Optimal Solution.

Let's take the example of the Fibonacci sequence where each number is the sum of the two preceding ones. Without Dynamic Programming, this calculation can be very time-consuming due to repeated computations. However, with Dynamic Programming, once a certain Fibonacci number is computed, it is stored for later use, thus reducing its time complexity significantly.

function fib(n) {
let table = Array(n+1).fill(0);
table[1] = 1;
for(let i=2; i<=n; i++)
table[i] = table[i-1] + table[i-2];
return table[n];
}


The beauty of Dynamic Programming is that it's an excellent example of a 'divide and conquer' technique. It leverages the power of previous computations to facilitate new ones. This characteristic often leads to exponential improvements in resource usage for many problems in computer science.

## Exploring Examples of Dynamic Programming

Dynamic Programming, with its efficiency and computational power, finds numerous applications in real-world problems. From straightforward sequence problems like the Fibonacci series to complex computational problems such as shortest-path algorithms, and even in state-of-the-art machine learning algorithms, Dynamic Programming plays a pivotal role.

### Practical Dynamic Programming Examples Shared

Let's delve into some practical examples to showcase the power of Dynamic Programming. These include the iconic Fibonacci sequence, the shortest path problem, and the longest common subsequence problem.

Fibonacci Sequence: The Fibonacci sequence is a series of numbers where each number is the sum of the two preceding ones. It's one of the simplest and most famous examples of a problem that can be solved more optimally with Dynamic Programming.

Without Dynamic Programming, calculating the nth Fibonacci number has a time complexity of $$O(2^n)$$.

function fib(n) {
if(n<=1)
return n;
else
return fib(n-1) + fib(n-2);
}

However, with dynamic programming, the time complexity reduces to $$O(n)$$ after trading time complexity for space complexity:
function fibDP(n) {
let fibArray = [0, 1];
for(let i=2; i<=n; i++)
fibArray[i] = fibArray[i-1] + fibArray[i-2];
return fibArray[n];
}


Shortest Path Problem: Finding the shortest or least-cost path from a starting point to an end point through a graph of interconnected nodes is optimised using Dynamic Programming. Algorithms such as Dijkstra's and Bellman-Ford leverage the principles of Dynamic Programming.

Longest Common Subsequence: Identifying the length of the longest subsequence that two sequences have in common. It's a popular computational problem solvable by Dynamic Programming.

#### Dynamic Programming Tables: A Simplified Approach

Dynamic Programming utilises a table to store the solutions to solved subproblems. This approach is best illustrated when solving the Longest Common Subsequence (LCS) problem. Here, a 2-dimensional table is created with one sequence represented along the rows and the other sequence along the columns.

Let's find the LCS of two sequences, 'ABCBDAB' and 'BDCAB'. First, create a table with the additional first row and column filled with zeroes. Subsequent entries use the formula: $LCS(i, j) = \begin{cases} LCS(i-1, j-1) + 1 &\quad\text{if } sequence1[i] = sequence2[j]\\ max(LCS(i, j-1), LCS(i-1, j)) &\quad\text{otherwise}\\ \end{cases}$ The resulting table can be represented as follows:

 0 0 0 0 0 0 0 1 1 1 1 1 0 1 1 2 2 2 0 1 2 2 2 2 0 1 2 2 2 3 0 1 2 2 3 3 0 1 2 2 3 4 0 1 2 3 3 4
Here, the last cell of the last row signifies the maximum length of the common subsequence being '4', and the common subsequence is 'BCAB'.

Formulating proper Dynamic Programming solutions often requires a significant amount of practice and careful observation. The ability to define the state and formulate the recursion relation is vital in successfully designing and implementing solutions with Dynamic Programming.

## The Differentiation Between Dynamic and Linear Programming

Dynamic Programming (DP) and Linear Programming (LP) are two powerful mathematical and computational techniques used to solve optimisation problems. While they share some similarities like their ability to handle complex problems efficiently, clear differentiation exists between their methodologies and problem-solving approaches.

### Highlighting the Fundamental Differences

Dynamic Programming and Linear Programming, despite the shared term 'programming', hold distinct perspectives when it comes to problem-solving. Understanding these differences can assist you in selecting the best technique to apply when tackling various complex computational dilemmas.

Dynamic Programming : Emphasises the concept of overlapping subproblems, solving each one separately and storing its solution for future reference. It builds up the answer to the larger problem by synthesising these stored results. It's perfect for solving problems that exhibit optimal substructure property, that is, an optimal solution to the main problem can be constructed from optimal solutions of its subproblems.

Linear Programming : Concentrates on maximising or minimising linear functions, subject to linear constraints. It involves a mathematical model where it represents the decision-making problem as linear relationships. Unlike Dynamic Programming, Linear Programming doesn't necessarily factor in previous decisions to influence the current one.

• Overlapping Subproblems: Dynamic Programming excels in problems with overlapping subproblems, while Linear Programming doesn't consider this aspect.
• Constraints: Linear Programming aims to find the optimal solution within specified linear constraints, while Dynamic Programming derives its flexibility from not being rigidly bound by such constraints.
• Approach: While Dynamic Programming is essentially a recursion plus memoisation approach, Linear Programming adopts a purely mathematical approach.
• Optimisation: Yet another critical point is optimisation. Linear Programming is better suited to large scale problems where optimisation is a priority, while Dynamic Programming is ideal for breaking down intricate problems into simpler, solvable parts.

#### How Dynamic Programming Differs From Linear Programming?

Dynamic Programming and Linear Programming serve different purposes, and being clear on their core differences can help you decide which technique to use when faced with a particular problem.

Consider a problem of maximising profits in a manufacturing scenario. If you're looking at the production variables and want to maximise profit given certain constraints like raw material availability, labour, and capital, Linear Programming would be the recommended method. However, suppose you're solving a problem where decisions made at one stage influence the options available at the next stage, such as optimising inventory in a warehouse over multiple time periods. In that case, Dynamic Programming would be ideal because it uses memoisation to store and utilise past information.

At a deeper level, both Linear Programming and Dynamic Programming are about strategic decisions and optimisation. However, their methodologies separate them. While Linear Programming provides a snapshot approach, operating over a given set of linear constraints to yield the best outcome, Dynamic Programming takes a process view of decision-making, where decisions at one stage influence the choices available at each subsequent stage. Making the right choice between both depends entirely on the problem at hand.

In a nutshell, the principal difference between Dynamic Programming and Linear Programming lies within their focus and approaches. Dynamic Programming is a method that's well-suited for problems that demand consideration of previous decisions, while Linear Programming is adept at handling problems requiring optimisation within linear constraints.

## Diving Into Specific Strategies of Dynamic Programming

Dynamic Programming is renowned for its effectiveness in breaking down complex problems into manageable chunks and solving each subproblem only once, saving both time and computational resources. Its versatility extends beyond standard optimisation problems, reaching into the realms of game theory and decision analysis. Two key strategies in this context are Minimax and Maximin, fundamental strategies employed in decision-making processes under uncertainty. These play a particularly important role when employing Dynamic Programming.

### The Concept of Minimax and Maximin in Dynamic Programming

These two concepts revolve around optimising the worst-case scenario. When facing a decision with multiple possible outcomes, these strategies provide a way to navigate through uncertainty by focussing on the best of the worst possible outcomes.

Minimax Strategy: This strategy seeks to minimise the maximum possible loss. That is, out of all possible worst-case scenarios, the Minimax strategy aims to find the decision that has the smallest loss. It's typically used in two-player zero-sum games and decision-making under uncertainty.

Maximin Strategy: Conversely, the Maximin strategy strives to maximise the minimum gain. Among all the best possible outcomes, the Maximin strategy targets the decision yielding the maximum gain. Similar to Minimax, it finds applications in game theory and decision-making under uncertainty.

Fairly seen, both concepts aim to deal with uncertainty in decision-making, but from slightly different perspectives. On the one hand, Minimax seeks to avoid the worst of the worst outcomes, while on the other hand, Maximin aims to achieve the best of the best possible result.

• Focus: While Minimax focuses on losses, Maximin is centred on gains.
• Strategy Nature: Minimax is pessimistic and risk-averse, whereas Maximin entails a more optimistic and risk-seeking approach.
• Decision: The Minimax principle always chooses the action that minimises the maximum regret, whereas the Maximin principle attempts to maximise the minimum profit.

#### How Dynamic Programming Solves Minimax and Maximin Problems?

While game theory might first come to mind with the Minimax and Maximin strategies, these concepts also find themselves at home in Dynamic Programming. Through its unique problem-splitting methodology, Dynamic Programming provides a systematic and efficient way to solve decision-making problems that hinge on these strategies.

Let's think about a simple game, wherein you can choose a number between 1 and 10, and your opponent can do the same following your choice. The player picking the highest number wins, and the loser pays the winner an amount equal to the difference between the two numbers. The game ends when a player runs out of funds. This is a zero-sum game, meaning one player's gain is the other's loss. By applying the Minimax strategy through dynamic programming, you aim to minimise your maximum possible payment in each round.


function minimax(gameState) {
let bestMove;
let bestScore = Infinity;

for (let move of gameState.getLegalMoves()) {
gameState.makeMove(move);
let score = max(gameState);
gameState.undoMove(move);

if (score < bestScore) {
bestScore = score;
bestMove = move;
}
}
return bestMove;
}


In the above code, the minimax function looks at every possible move at the current state. For each move, it advances the state and computes the maximum possible loss if the opponent played optimally, then reverts to the original state. Finally, it keeps track of the move that gives the lowest maximum loss, which is the optimal move according to the Minimax strategy.

Despite its simplicity, the Minimax strategy brought life to crucial algorithms in game theory and computer science. The design philosophy behind Minimax beautifully harmonises with Dynamic Programming techniques. By breaking a multi-stage problem down into several single-stage problems, then solving these one by one, what could initially seem like an impossible problem becomes a straightforward computational task.

Synopsis: Whether it's a zero-sum game or decision-making under uncertainty, Minimax and Maximin provide effective strategies to navigate these situations. Coupled with the efficient problem-solving capabilities of Dynamic Programming, they offer a potent tool in the analysis of decision-making problems.

## Expanding Your Knowledge on Dynamic Programming

Dynamic Programming is a powerful mathematical technique used in the field of optimisation, enabling you to break down complex problems into simpler sub-problems, solve each individually, and use these solutions to find the solution to the original problem. This bottom-up approach is particularly effective in problems which exhibit the property of overlapping sub-problems.

### Extra Insights on Dynamic Programming Method

Dynamic Programming adopts a methodical approach to problem-solving. Beginning with an initial state, it examines various sub-problems in a careful order, each time picking the best choice available until the final solution is reached. This is what distinguishes Dynamic Programming from other problem-solving methods: its ability to store the results of distinct sub-problems computed, often termed memoisation.

Memoisation: This technique involves storing the solutions of expensive function calls and reusing them when the same inputs occur again, instead of recomputing them. In Dynamic Programming, this drastically reduces the computational time because the sub-problems do not have to be solved multiple times.

Dynamic Programming operates under the principle that the optimal solution to a problem can be determined by finding the optimal solutions to its sub-problems. This property is known as optimal substructure and is one of the defining characteristics of problems suitable for Dynamic Programming.

Optimal Substructure: A problem has an optimal substructure if an optimal solution to the larger problem can be constructed from optimal solutions of its sub-problems. For a problem to be solved with Dynamic Programming, it must exhibit this property.

• Pattern of Overlapping Sub-problems: Dynamic Programming shows its might in problems containing overlapping sub-problems. These are problems where bigger problem instances can be resolved by efficiently solving the smaller instances.
• Breakdown Approach: Dynamic Programming chooses a bottom-up approach. It solves all the sub-problems first, then builds up solutions to larger and larger sub-problems.
• Utilising Past Knowledge: Unlike other methodologies that solve a problem from scratch, Dynamic Programming uses the information of already solved sub-problems to expedite the process.

#### How Dynamic Programming is Utilised in Mathematical Problems?

Dynamic Programming unlocks new dimensions in solving mathematical problems, particularly in optimisation problems. It deciphers the problem in stages, one step at a time, storing the results of each stage to utilise in the next.

Take the famous problem of the 'Travelling Salesman'. The problem lies in finding the shortest possible route for a salesman who needs to visit a specific set of cities once and return to the original city. Using Dynamic Programming, we can start by examining the smaller sub-problems, for instance, the shortest route among three cities, and gradually build up to the larger problem. This problem has an exponential number of solutions in the number of cities, but through Dynamic Programming, the computation time can be significantly reduced.

Applying Dynamic Programming to mathematical problems often requires determining the optimal substructure of the problem and implementing memoisation to store the results of subproblems for later use. Another crucial aspect is defining the recursive relation between sub-problems, often sequenced by a process of trial-and-error and based on intuition and experience.

function fibonacci(n, memo) {
if (memo[n] !== undefined) return memo[n];
if (n <= 2) return 1;
return memo[n] = fibonacci(n-1, memo) + fibonacci(n-2, memo);
}


In the above code snippet, we see an optimised function for the Fibonacci sequence using Dynamic Programming. By passing the 'memo' object that stores previously calculated values, the function prevents repetitive computations and significantly reduces the total computations.

The potential applications of Dynamic Programming in mathematics are wide-ranging, whether it's calculus, algebra, programming, operations research, or artificial intelligence. By transforming and sequencing a mathematical problem into manageable components via Dynamic Programming, you can delve into complex calculations and predictions with relative ease.

One key takeaway from all this is the profound shaping role Dynamic Programming plays in the world of mathematics. From computational algorithms to advanced scientific computations, Dynamic Programming brings a fresh, efficient, and resource-saving perspective to how mathematical problems are solved.

## Dynamic Programming - Key takeaways

• Dynamic Programming: A technique used to optimise calculations by breaking a problem down into simpler sub-problems, solving each separately, and storing the solutions for later use.
• Fibonacci Sequence: An example of a problem that can be solved more efficiently using Dynamic Programming, with time complexity reduced from $$O(2^n)$$ to $$O(n)$$.
• Shortest Path Problem and Longest Common Subsequence: Two examples of problems that can be optimised using Dynamic Programming.
• Dynamic Programming Tables: An approach that uses a table to store the solutions of solved subproblems, often used when dealing with the Longest Common Subsequence problem.
• Difference between Dynamic and Linear Programming: Dynamic Programming excels in problems with overlapping subproblems and doesn't necessarily have strict constraints. On the other hand, Linear Programming aims to find the optimal solution within specified linear constraints, without considering overlapping subproblems.
• Minimax and Maximin Strategies: Two key strategies applied in Dynamic Programming that are used in decision-making processes. Minimax strategy seeks to minimise the maximum loss, while Maximin strategy aims to maximise the minimum gain.

#### Flashcards in Dynamic Programming 19

###### Learn with 19 Dynamic Programming flashcards in the free StudySmarter app

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What are the primary steps to follow in the process of dynamic programming?
The primary steps in dynamic programming are: 1) Characterise the structure of an optimal solution. 2) Define the value of an optimal solution recursively in terms of smaller subproblems. 3) Compute the value of an optimal solution in a bottom-up fashion. 4) Construct an optimal solution to the problem from the computed information.
What is the fundamental principle behind dynamic programming in mathematics?
The fundamental principle behind dynamic programming is to simplify a complex problem by breaking it down into smaller sub-problems. These sub-problems are solved individually and their solutions are stored for future reference to avoid repetition, thus optimising the overall solution process.
How can dynamic programming be utilised to solve complex mathematical problems?
Dynamic programming can be used to solve complex mathematical problems by breaking them down into simpler, overlapping sub-problems, solving each of these sub-problems only once, and storing their results in a table. This approach reduces computation time and solves problems efficiently by avoiding repetition of the same computation.
Is there a specific case where dynamic programming proves more efficient than other mathematical methodologies?
Yes, dynamic programming proves more efficient in cases where there are overlapping subproblems and optimal substructure. This methodology reduces computational time by storing and reusing subproblem solutions instead of recomputing them, making it more efficient for problems like knapsack and shortest path algorithms.
Can dynamic programming be employed for solving problems in other disciplines besides mathematics?
Yes, dynamic programming can be employed in various disciplines like computer science, economics, operations research, and bioinformatics, for solving optimisation problems, decision problems, and resource allocation problems.

## Test your knowledge with multiple choice flashcards

How is the optimal action for a given state denoted in a dynamic programming table?

What is a minimax problem?

What is a maximin problem?

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