The GRT Planning System: Backward Heuristic Construction in Forward State-Space Planning

This paper presents GRT, a domain-independent heuristic planning system for STRIPS worlds. GRT solves problems in two phases. In the pre-processing phase, it estimates the distance between each fact and the goals of the problem, in a backward direction. Then, in the search phase, these estimates are used in order to further estimate the distance between each intermediate state and the goals, guiding so the search process in a forward direction and on a best-first basis. The paper presents the benefits from the adoption of opposite directions between the preprocessing and the search phases, discusses some difficulties that arise in the pre-processing phase and introduces techniques to cope with them. Moreover, it presents several methods of improving the efficiency of the heuristic, by enriching the representation and by reducing the size of the problem. Finally, a method of overcoming local optimal states, based on domain axioms, is proposed. According to it, difficult problems are decomposed into easier sub-problems that have to be solved sequentially. The performance results from various domains, including those of the recent planning competitions, show that GRT is among the fastest planners

Symmetry reduction and heuristic search for error detection in model checking

The state explosion problem is the main limitation of model checking. Symmetries in the system being verified can be exploited in order to avoid this problem by defining an equivalence (symmetry) relation on the states of the system, which induces a semantically equivalent quotient system of smaller size. On the other hand, heuristic search algorithms can be applied to improve the bug finding capabilities of model checking. Such algorithms use heuristic functions to guide the exploration. Bestfirst is used for accelerating the search, while A* guarantees optimal error trails if combined with admissible estimates. We analyze some aspects of combining both approaches, concentrating on the problem of finding the optimal path to the equivalence class of a given error state. Experimental results evaluate our approach

TALplanner in IPC-2002: Extensions and Control Rules

TALplanner is a forward-chaining planner that relies on domain knowledge in the shape of temporal logic formulas in order to prune irrelevant parts of the search space. TALplanner recently participated in the third International Planning Competition, which had a clear emphasis on increasing the complexity of the problem domains being used as benchmark tests and the expressivity required to represent these domains in a planning system. Like many other planners, TALplanner had support for some but not all aspects of this increase in expressivity, and a number of changes to the planner were required. After a short introduction to TALplanner, this article describes some of the changes that were made before and during the competition. We also describe the process of introducing suitable domain knowledge for several of the competition domains

Passage, Profit, Protection and the Challenge of Participation

Accepting that successful 'development' is premised on a population's participation in a collective undertaking, we must understand urban residents' interactions and ambitions. In African cities being transformed by geographic and social mobility, it is umigration, urbanization, African cities, social cohesion, integration,

The Benefits Planner: Welcome to The Benefits Planner

This newsletter will provide valuable information on how work for persons with disabilities effects government benefits, with an emphasis on the Supplemental Security Income (SSI) and Social Security Disability Insurance (SSDI) work incentives. Each newsletter will contribute to an ongoing dialogue on topics related to benefits and work

Gated ‘communities’ - their lifestyle versus urban governance

Today there is a widespread fear of crime on a global scale. This can be seen as a response to social inequalities, social polarisation and the fragmentation of cities, which has been caused by neo-liberalism. Worldwide, an increasing number of higher income groups have looked to security measures, such as cameras, fences, walls and gates, to separate themselves from other people in the city. These physical measures, in combination with hired guards, replace the ‘older’ social control mechanisms, which are based on social cohesion within the community concerned. One may question whether those living in gated ‘communities’ indeed feel responsible for other urbanites. In other words, will such a hard closure (physically-marked segregation) lead to soft closure, reflected in social-cultural and political segregation. What is the impact of the lifestyle(s) of those living in gated communities on the dynamics of the city, urban identity and urban governance?