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  3. Vrije Universiteit Amsterdam (VU)
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  5. Intelligent Systems

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[ Intelligent Systems ] Summary | Lecture Notes

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These are lecture notes of Intelligent Systems from the study Lifestyle Informatics/Artificial Intelligence at VU Amsterdam. Everything marked in yellow is extra important, everything that's blue is an example.

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  • 3 april 2018
  • 17
  • 2017/2018
  • Samenvatting

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Voorbeeld van de inhoud

INTELLIGENT SYSTEMS
LECTURE 1 – INTRODUCTION
MACHINE AS AN INTELLIGENT AGENT

Main methods to optimize for the best expected outcome: Text/image processing, data integration,
knowledge representation, automatic reasoning, machine learning and genetic programming (writing code to
improve its own behavior).

Philosophical questions: What is consciousness/free will? Can a machine be creative/have rights?
Ethical questions: surveillance/privacy, responsibility, accountability, using/misusing IS.

CORE TOPICS
• Topic 1: Search and heuristics (principal of rationality)
• Topic 2: Knowledge
• Topic 3: Adaptivity (machine learning)


STATE SPACE REPRESENTATION

Representing problems as state spaces problems (because the world is absurdly complex) à state spaces must
be abstracted/simplified for problem solving à (Abstract) solution = set of real paths that are solutions of the
real world.
1. What actions and states to consider, given a goal:
o What are appropriate states and initial states? (atomic)
o What are the possible actions to move between states? (atomic)
o What is the cost of an action?
2. Goal formulation: what are the successful world states?
3. Search: determine possible sequences of actions that lead to goal states and choose the “best”
sequence.
4. Execute: Give the solution and perform the actions.

Example - Romania:
⁃ Concrete problem: In Arad, flight from Bucharest.
o States: various (representation of) cities
o Actions: drive between cities
⁃ Concrete goal: Be in Bucharest
⁃ Find solution: sequence of best states (Arad à Sibiu à
Fagaras à Bucharest)
⁃ Execute plan: buy tickets + travel

SEARCH

Search through explicit tree generation:
1. ROOT = initial state
2. Nodes and leafs generated through successor function (related to action)

State = a (representation of) a physical configuration
Node = a data structure belonging to a search tree




1

,SEARCH STRATEGIES
Strategy = picking order of node expansion. Performance is measured in 4 ways:
1. Completeness: does it always find a solution if one exists?
2. Optimality: does it always find the least-cost solution?
3. Time Complexity: number of nodes generated/expanded?
4. Space Complexity: number of nodes stored in memory during search?
a. Time & Space complexity are measured in terms of problem difficulty:
i. b – maximum branching factor of the search tree (amount of possible choices)
ii. d – depth of the least-cost solution (best solution: best distance between root)
iii. m – maximum depth of the state space (may be infinite)


UNINFORMED SEARCH STRATEGIES

Uninformed search strategies (a.k.a. blind search) = use only information available in problem definition:
• Breadth-first search = expand SHALLOWEST unexpanded node
o Implementation: fringe is a FirstInFirstOut (FIFO) queue (A à B à C à D à E à F…)
§ Completion: Yes, if b is finite
§ Time complexity: Yes
§ Space complexity: Yes, if each node is retained in memory
§ Optimality: Yes, unless actions have different costs
• Disadvantage: longest path
• Depth-first search = expand DEEPEST unexpanded node
o Implementation: fringe is a LIFO queue (= stack) (A à B à D à H & I à E à J & K à C…)
§ Completion: No, unless search space is finite and no loops are possible
§ Time complexity: Yes, if many solutions, a bit faster than BF
§ Space complexity: Yes, backtracking search uses even less memory
§ Optimality: No, unless search space is finite and no loops are possible
• Depth-limited search = DF-search with depth limit l
§ Completion: Yes (if I >= d)
§ Optimality: No
• Depth-first Iterative Deepening search = Breadth-F + Depth search
§ Completion: Yes (no infinite paths)
§ Space Complexity: Yes
§ Optimality: Yes
• Bidirectional search
• Uniform-cost search




2

, LECTURE 2 – INFORMED & ADVERSARIAL SEARCH
INFORMED SEARCH (HEURISTICS)

General approach of informed search: best-first search (node is selected for expansion based on an evaluation
function f(n). Fringe = queue sorted in decreasing order of desirability.

Heuristic function = educated guess that reduces or limits the search for solutions in domains that are difficult
and poorly understood. Describes the expected quality of a node.
• h(n) = estimated cost of the cheapest path from node n to goal node.
• If n is goal, then h(n) = 0.
• Complete knowledge: often not available & too expensive to calculate
• Use partial knowledge/human expertise = educated guess (can be completely wrong)

Two elements, h1(n) and h2(n), with 0 < h1(n) < h2(n) < h *(n), then h2(n) is more informed than h1(n).
• Perfect heuristics: h(n) = h*(n)
• Trivial heuristics: h(n) = 0
o Advantage: With h1 fewer nodes have to be searched with h2
o Disadvantage: h2 is often more expensive to calculate.


ADVERSARIAL SEARCH

Games are a form of multi-agent environment. Competitive multi-agent environments give rise to adversarial
problems a.k.a. games. Easy to represent and agents restricted to small number of actions.


SEARCH – NO ADVERSARY
• Solution is (heuristic) method for finding goal
• Heuristics and CSP techniques can find optimal solution
• Evaluation function: estimate cost from start to goal
• Examples: path planning, scheduling activities

GAMES – ADVERSARY
• Solution is strategy (strategy specifies moves for every possible opponent reply)
• Time limits force an approximate solution
• Evaluation function: evaluate “goodness” of game position
• Examples: chess, checkers, Othello, backgammon, GO

Deterministic: you know where the figures are.
Chance: you know where the figures are, but there is always an element of chance.

MINMAX

• Two players: MAX and MIN
• MAX moves first (uses search tree to determine next move), and they take turns
until the game is over. Winner à reward & loser à penalty.
• Games as search
o Initial state: e.g. board configuration of chess
o Successor function: list of (move, state) pairs specifying legal moves
o Terminal test: is the game finished?
o Utility function: gives numerical value of terminal states
§ win (+1), lose (-1), draw (0) in tic-tac-toe




3

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