{"type":"ScholarlyArticle","title":"Delete- and Ordering-Relaxation Heuristics for HTN Planning","authors":[{"name":"Daniel Höller","slug":"daniel-holler","given":"Daniel","family":"Höller","affiliation":"Saarland University , Saarland Informatics Campus; Ulm University , Institute of Artificial Intelligence"},{"name":"Pascal Bercher","slug":"pascal-bercher","given":"Pascal","family":"Bercher","affiliation":"The Australian National University , College of Engineering and Computer Science; Ulm University , Institute of Artificial Intelligence"},{"name":"Gregor Behnke","slug":"gregor-behnke","given":"Gregor","family":"Behnke","affiliation":"University of Freiburg; Ulm University , Institute of Artificial Intelligence"}],"abstract":"In HTN planning, the hierarchy has a wide impact on solutions. First, there is (usually) no state-based goal given, the objective is given via the hierarchy. Second, it enforces actions to be in a plan. Third, planners are not allowed to add actions apart from those introduced via decomposition, i.e. via the hierarchy. However, no heuristic considers the interplay of hierarchy and actions in the plan exactly (without relaxation) because this makes heuristic calculation NP-hard even under delete relaxation. We introduce the problem class of delete- and ordering-free  HTN planning as basis for novel HTN heuristics and show that its plan existence problem is still NP-complete. We then introduce heuristics based on the new class using an integer programming model to solve it.","doi":"10.24963/ijcai.2020/564","url":"https://ijcai.org/proceedings/2020/564","pdf":"https://ijcai.org/proceedings/2020/0564.pdf","markdown":"https://ijcai.org/proceedings/2020/564.md","bibtex":"https://ijcai.org/proceedings/2020/bibtex/564","volume":{"slug":"2020","title":"Proceedings of the Twenty-Ninth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2020","url":"https://ijcai.org/proceedings/2020/","year":2020,"location":"Yokohama","dates":"January 2021 (postponed from 11-17 July 2020)","editors":"Christian Bessiere","isbn":"978-0-9992411-6-5","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2020"},"section":"Planning and Scheduling","track":"Main track","number":564,"pages":{"first":4076,"last":4083},"keywords":["Planning and Scheduling: Hierarchical planning","Planning and Scheduling: Search in Planning and Scheduling"],"videos":[{"label":"Video","url":"https://videos.ijcai20.org/24656/file_24656.m3u8","length_sec":282,"provider":"hls"}],"references":[{"position":1,"raw":"[Alford et al., 2009] Ron Alford, Ugur Kuter, and Dana S. 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