# Questions tagged [descriptive-complexity]

Descriptive complexity classifies problems based on how hard it is to express the problem in some logical formalism.

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### Is there a logic without induction that captures much of P?

The Immerman-Vardi theorem states that PTIME (or P) is precisely the class of languages that can be described by a sentence of First-Order Logic together with a fixed-point operator, over the class of ...
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I recently started reading about Descriptive Complexity, the branch of Complexity Theory studying the logic languages needed to express complexity classes. The main milestone in the area seems to be ...
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### Are there descriptive complexity representations of quantum complexity classes?

The title more or less says it all, but I guess I could add a bit of background and some specific examples I'm interested in. Descriptive complexity theorists, such as Immerman and Fagin, have ...
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### Descriptive complexity characterization of TimeSpace classes

Are there descriptive complexity characterizations for TimeSpace complexity classes like $\mathsf{SC^i}= \mathsf{DTimeSpace}(n^{O(1)},O(\lg^i n))$?
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### P and Descriptive Complexity

In the Complexity Zoo, it says [1] that, in descriptive complexity, $P$ can be defined by three different kind of formulae, $FO(LFP)$ which is also $FO(n^{O(1)})$, and also as $SO(HORN)$. However, ...
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### What do dichotomy theorems feed on?

It is well known that certain classes of NP-problems have dichotomy theorems, which guarantee that every task in the class is either NP-complete or is in P. The best known such result is Schaefer's ...
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### Descriptive Complexity characterzation of BPP

We know of descriptive complexity characterizations of classes such as P, and NP, which use First Order logic, and operators. Does BPP have a characterization under descriptive complexity, too(any ...
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### Collapsing of exptime and alternation bounded turing machine

This question was already asked on math overflow, but I did not find any answer to my question (or let say the answer was that up to the knowledge of those people, no answer were known) Let C be a ...
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### How can we express “$P=PSPACE$” as a first-order formula? [closed]

How can we express "$P=PSPACE$" as a first-order formula? Which level of the arithmetic hierarchy contains this formula (and what is the currently known minimum level of the hierarchy that contains it)...
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### Non-interesting numbers via resource-bounded properties?

There is an old joke about the smallest non-interesting number being interesting in itself (I have heard it attributed to Richard Hamming). This is then used to justify the argument that every number ...
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### Expressiveness of Infinitary Logic

I'm trying to put together a general picture of the expressiveness of some logics: First-Order Logics, Fixed-Point Logics, (Finite Variable) Infinitary Logics and the respected versions with Counting. ...
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### Nonterminal descriptional complexity of regular languages

Recently I became interested in grammar complexity of regular language. Prior to searching for literature, I tried to investigate it on my own, proving two lemmas from comment below. I am aware of an ...
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### Logic capturing automorphism-invariant $\mathsf{AC^0}$ properties

Q1. Is there a logic that is computable in polynomial-time which contains all order-invariant properties computable in smaller classes like $\mathsf{AC^0}$ (or $\mathsf{TC^0}$)? Motivation As you ...
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### Context-free Grammar for a Context-free Language Intersecting a Regular Language (get the Maximum Number of Rules)

It is well known that the intersection of $L \cap R$ of a context-free Language $L$ and a regular Language $R$ is context-free. Each proof I have seen constructs a automaton (a PDA) that accepts $L$ ...