V. G. Durnev. Positive theory of a free semigroup. Soviet Math. Doklady, 211(4):772--774, 1973. Home/Search Document Not in Database Summary Related Articles Check |
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Equational Theory of Context Unification is Hard - Vorobyov (1998) (Correct)
....enumerable sets. For comparison, the following undecidability results are known about quantified fragments of context unification. Quine [10] showed that the full firstorder theory of free semigroups is undecidable (this corresponds to context unification in unary signatures) Durnev [3] improved it to the undecidability of 989 3 positive (without negation, but with and ) theory of free semigroups. Marchenkov [8] improved it to the undecidability of 89 4 positive theory of free semigroups. Durnev [2] improved it to undecidability of 89 3 positive theory of free ....
V. G. Durnev. Positive theory of a free semigroup. Soviet Math. Doklady, 211(4):772--774, 1973.
Equational Theory of Context Unification is Hard - Vorobyov (1998) (Correct)
....of formulas, where Pi 0 1 is the class of all co recursively enumerable sets. For comparison, the following undecidability results are known about quantified fragments of context unification: 1. Quine 1946) showed that the full first order theory of free semigroups 1 is undecidable. 2. (Durnev 1973) improved it to undecidability of 989 3 positive (without negation, but with and ) theory of free semigroups. 3. Marchenkov 1982) improved it to undecidability of 89 4 positive theory of free semigroups. 4. Durnev 1997) improved it to undecidability of 89 3 positive theory of free ....
Durnev, V. G. (1973), `Positive theory of a free semigroup', Soviet Math. Doklady 211(4), 772--774.
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