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In formal language theory, in particular in algorithmic learning theory, a class C of languages has finite thickness if every string is contained in at most finitely many languages in C. This condition was introduced by Dana Angluin as a sufficient condition for C being identifiable in the limit.

The related notion of M-finite thickness

Given a language L and an indexed class C = { L₁, L₂, L₃, ... } of languages, a member language L_j ∈ C is called a minimal concept of L within C if L ⊆ L_j, but not L ⊊ L_i ⊆ L_j for any L_i ∈ C. The class C is said to satisfy the MEF-condition if every finite subset D of a member language L_i ∈ C has a minimal concept L_j ⊆ L_i. Symmetrically, C is said to satisfy the MFF-condition if every nonempty finite set D has at most finitely many minimal concepts in C. Finally, C is said to have M-finite thickness if it satisfies both the MEF- and the MFF-condition.

Finite thickness implies M-finite thickness. However, there are classes that are of M-finite thickness but not of finite thickness (for example, any class of languages C = { L₁, L₂, L₃, ... } such that L₁ ⊆ L₂ ⊆ L₃ ⊆ ...).

References

1. Dana Angluin (1980). "Inductive Inference of Formal Languages from Positive Data" (PDF). Information and Control. 45 (2): 117–135. doi:10.1016/s0019-9958(80)90285-5. (citeseer.ist.psu.edu); here: Condition 3, p.123 mid. Angluin's original requirement (every non-empty string set be contained in at most finitely many languages) is equivalent.
2. Andris Ambainis; Sanjay Jain; Arun Sharma (1997). "Ordinal mind change complexity of language identification". Computational Learning Theory (PDF). LNCS. Vol. 1208. Springer. pp. 301–315.; here: Definition 25
3. Ambainis et al. 1997, Definition 26
4. Ambainis et al. 1997, Corollary 29

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