We prove that the problem STO of deciding whether or not a finite set E of term equations is subject to occur-check is in NP. E is subject to occur-check if the execution of the Martelli-Montanari unification algorithm gives for input E a set E โช {x = t}, where t = x and x appears in t. Apt et al. (
โฆ LIBER โฆ
The shortest common nonsubsequence problem is NP-complete
โ Scribed by M. Middendorf
- Publisher
- Elsevier Science
- Year
- 1993
- Tongue
- English
- Weight
- 337 KB
- Volume
- 108
- Category
- Article
- ISSN
- 0304-3975
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A finite set of term equations \(E\) is called subject to the occur-check (STO) if a sequence of actions of the Martelli-Montanari unification algorithm starts with \(E\) and ends with a failure due to occur-check. We prove here that the problem of deciding whether \(E\) is STO is NP-hard.