1. Tabu Search
Review:
Branch and bound has a “rigid” memory structure (i.e. all branches are completed or fathomed).
Simulated Annealing has no memory structure. Relies on randomization to search the solution space.

2. Tabu Search Primary Features of Tabu Search:
Adaptive memory - remembers features of good/bad solutions that you encounter).
Responsive exploration – exploration based on past exploration.

3. Tabu Search Basic Algorithmic Features:
Always move to the best available neighborhood solution, even if it is worse than the current solution.
Maintain a list of solution points that must be avoided (not allowed) or a list of move features that are not allowed. Referred to as the tabu list.
Update the tabu list based on some memory structure (short-term memory). Remove tabu moves after some time period has elapsed (tenure).
Allow for exceptions from the tabu list (aspiration criteria).
Expand the search area, modify tenure or size of tabu list.

4. Tabu Search Tabu Search Short-Term Memory Component (F. Glover, 1990):
Begin with a Starting Current Solution
Obtain the solution from initialization or from
An intermediate or long-term memory component
Create a Candidate List of Moves
(If applied, each move would generate
a new solution from the current solution)
Choose the Best Admissible Candidate
(Admissibility is based on the tabu restrictions and aspiration criteria.)
Designate the solution obtained as the new current solution.
Record it as the new Best Solution if it improves on the previous best.
Stopping Criterion
Stop if a specified number of iterations has elapsed
In total or since the last Best Solution was found.
Stop
Continue
Terminate Globally or Transfer
A transfer initiates an intensification or
Diversification Phase embodied in an
intermediate or long-term memory component.
Update Admissibility Conditions
Update tabu restrictions and
Aspiration criteria.

5. Tabu Search Issue 1: What is a tabu list? How is it generated?
How does is it incorporate memory?
See handout.
Issue 2:
How can a tabu list be used to expand the search area?

6. Tabu Search Issue 3: What is an aspiration criteria?
Criteria which allows a tabu moved to be accepted under certain conditions.
Most common aspiration criterion, if the move finds a new best solution, then accept the move even if the move is tabu.

7. Tabu Search
Issue 4: Algorithm for selecting best admissible candidate (Glover).
Evaluate Each Candidate Move
Does the move yield a higher evaluation
Than any other move found admissible so far? NO
YES
Check Tabu Status
Is the move tabu?
YES NO
Check Aspiration Level
Does move satisfy
Aspiration criteria?
YES
Move is Admissible
Designate as best
Admissible candidate. NO
Candidate List Check
Is there a good probability of better
moves left, or should candidate list be extended?
YES NO
Make the Chosen Best Admissible Move

8. Tabu Search
Example of Tabu Search – See handouts