Abstract
We give recursion-theoretic characterizations of the counting class \(\textsf {\#P} \), the class of those functions which count the number of accepting computations of non-deterministic Turing machines working in polynomial time. Moreover, we characterize in a recursion-theoretic manner all the levels \(\{\textsf {\#P} _k\}_{k\in {\mathbb {N}}}\) of the counting hierarchy of functions \(\textsf {FCH} \), which result from allowing queries to functions of the previous level, and \(\textsf {FCH} \) itself as a whole. This is done in the style of Bellantoni and Cook’s safe recursion, and it places \(\textsf {\#P} \) in the context of implicit computational complexity. Namely, it relates \(\textsf {\#P} \) with the implicit characterizations of \(\textsf {FPTIME} \) (Bellantoni and Cook, Comput Complex 2:97–110, 1992) and \(\textsf {FPSPACE} \) (Oitavem, Math Log Q 54(3):317–323, 2008), by exploiting the features of the tree-recursion scheme of \(\textsf {FPSPACE} \).
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Notes
\(|\bar{x}|=(|x_1|,\dots ,|x_n|)\), where \(\bar{x}=(x_1,\dots ,x_n)\).
“It is well known that \(\textsf {\#P} \) is closed not only under addition and multiplication but also under summation of exponentially many \(\textsf {\#P} \) functions...” [8, p. 458].
Wagner writes \(k\textsf {\#P} \) for our notation \(\textsf {\#P} _k\).
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Acknowledgements
The first author is supported by the ERC CoG DIAPASoN GA 818616. The research of the third and main author is supported by national funds throught the FCT - Fundação para a Ciência e Tecnologia, I.P., under the scope of the project UIDB/00297/2020 (Center for Mathematics and Applications). The second author is supported by the same project and by the Udo Keller Foundation.
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Dal Lago, U., Kahle, R. & Oitavem, I. Implicit recursion-theoretic characterizations of counting classes. Arch. Math. Logic 61, 1129–1144 (2022). https://doi.org/10.1007/s00153-022-00828-4
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DOI: https://doi.org/10.1007/s00153-022-00828-4
Keywords
- \(\textsf {\#P} \)
- Counting hierarchy
- Implicit computational complexity
- Function algebras
- Recursion-theoretic characterizations