thanks for your help. This is my first question, so I am very sorry for the bad presentation of the question.

I am studying computer science and this is the question I have been asked for the course of Theory of computation and complexity

Edited question

1.For every two languages $A,B∈PSPACE$, there is a language $C∈PSPACE$ such that $A≤_PC$$B≤_PC$

2.For every two languages $A,B∈PSPACE$ , there is a language $C∈PSPACE$ such that $C≤_PA$$B≤_PC$

3.For every two languages $A,B∈PSPACE$, there is a language $C∈PSPACE$ such that $C≤_PB$$C≤_PA$

What I have tried is:

  1. Correct, we'll take $C∈PSPACE-Complete$ then $C∈PSPACE$ and every $L∈PSPACE$ is $L≤_PC$
    so in particular for languages $A,B∈PSPACE$ there $C≤_PB$$C≤_PA$

  2. Not correct , we'll take $B∈PSPACE-Complete$ then $B∈PSPACE$ , $A∉PSPACE-Complete$ and $A∈PSPACE$. if $C∈PSPACE$ and $C≤_PA$$B≤_PC$, then $C ∈PSPACE-Complete$ and $A ∈PSPACE-Complete$ , Contradiction to the the fact $A∉PSPACE-Complete$!

  3. Not correct , If A or B are trivial then there is no C that $C∈PSPACE$ such that $C≤_PB$$C≤_PA$

Can you please help me determine which of them are correct and which are not (can be several that are correct), and why?

  • $\begingroup$ What have you tried? Can you edit the question and use latex? $\endgroup$ Jan 31 at 18:59
  • 1
    $\begingroup$ Please ask only one question per post. I see three separate questions here. We are looking for posts that will be useful to others in the future. As such, a post that just contains an exercise-style task and a request for us to solve it for you, or a claim and a request for us to tell you whether it is true or not, is typically not a good fit for our site format. See here for tips on asking questions about exercise-style problems. $\endgroup$
    – D.W.
    Jan 31 at 20:05
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    $\begingroup$ Although I answered the question, I am downvoting it to discourage "help me" type of questions that show zero effort from the asker side. Also, honestly, I did not thought about answering, but I did after I saw others began answering, which is also not good because if the question stays, and I think I have a more informative answer, then I am kinda forced to answer too. $\endgroup$ Jan 31 at 20:07

2 Answers 2


Regarding 1: choose $C = \{0x \mid x \in A \} \cup \{1x \mid x\in B\}$.

Regarding $2$: $B \le_P \emptyset \implies B=\emptyset$ however $\{\varepsilon\} \le_P B \implies B \neq \emptyset$.

Regarding $3$: the statement is not well-formed.

  1. Correct: you can choose $C$ to be any $\text{PSPACE-complete}$ language.
  2. The only known counter-examples to this claim are ones where $C$ or $A$ are trivial, and hence, sadly, a counter-example would be one of these boring edge cases (I encourage you to check what happens if you consider $\emptyset$ or $\Sigma^*$). It is worth mentioning however that if $C$ and $A$ are assumed to be nontrivial, then the claim essentially suggests that languages in $\text{PSPACE}$ are in a sense dense w.r.t PTIME reductions, and I believe this was the intention behind the claim. Yet, we don't know whether this is true or false as the claim then is equivalent to $\text{P} = \text{PSPACE}$, which is open.
  3. The claim does not compile.

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