The helping hierarchy
RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 35 (2001) no. 4, pp. 367-377.

Schöning [14] introduced a notion of helping and suggested the study of the class P help (𝒞) of the languages that can be helped by oracles in a given class 𝒞. Later, Ko [12], in order to study the connections between helping and “witness searching”, introduced the notion of self-helping for languages. We extend this notion to classes of languages and show that there exists a self-helping class that we call SH which contains all the self-helping classes. We introduce the Helping hierarchy whose levels are obtained applying a constant number of times the operator P help (·) to the set of all the languages. We show that the Helping hierarchy collapses to the k-th level if and only if SH is equal to the k-th level. We give characterizations of all the levels and use these to construct a relativized world in which the Helping hierarchy is infinite.

Classification : 68Q15,  68Q05,  03D15
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     author = {Cintioli, Patrizio and Silvestri, Riccardo},
     title = {The helping hierarchy},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
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     publisher = {EDP-Sciences},
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     number = {4},
     year = {2001},
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     url = {http://www.numdam.org/item/ITA_2001__35_4_367_0/}
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Cintioli, Patrizio; Silvestri, Riccardo. The helping hierarchy. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 35 (2001) no. 4, pp. 367-377. http://www.numdam.org/item/ITA_2001__35_4_367_0/

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