I have been led to believe that the following statement

$X \in NP \land Y \not\in NP \implies X \not\le^m_p Y$

Is True.

But I am having difficult proving it. And I'm not even sure it IS true anymore.

At first it seemed reasonable that it was true. Since X is in NP there must exist some NP-complete problem Z that it reduces to and my thinking is that any other problem that X reduces to must also reduce to this NP-complete problem Z, that is, since X is in NP both it and any problem it can be reduced to must also reduce to an NP-complete problem Z so it cannot be the case that X can be reduced to a problem Y that is NOT in NP. This is not my proof but simply my intuition for why the statement is True.

I was hoping initially to prove it by contradiction:

suppose not, that is, suppose $X \in NP \land Y \not\in NP$ but $X \le^m_p Y$

since $X \in NP$ it must be that $\exists$ a poly-time verifier algorithm $B$ such that
$\forall$ inputs $x \in X$ : $\exists$ certificate $t$ s.t. $B(x,t) = yes$

and since $X \le^m_p Y$ it must be that $\exists$ a poly-time reduction $f$ such that
$\forall$ inputs $x$ $x \in X \iff f(x) \in Y$

it follows then that $f(x) \in Y \iff x \in X \iff \exists t B(x,t) = yes$

Then at this point I was hoping to reverse engineer the verifier B to come up with a poly-time verifier for Y thus contradicting the supposition that $Y \not\in NP$ similar to what the answer to this question does:
If X is polynomial reduction to Y and Y is in NP, then X is in NP?
But that direction doesn't really work and I haven't found way to use the reasoning above to try and show that $X \not\in NP$ and achieve contradiction that way

And so this is where I am stuck. Am I missing something or am I wrong in my intuition that the statement is true?


1 Answer 1


The time hierarchy theorem states that $\mathsf{NP}\subsetneq \mathsf{NEXP}$.

So if you consider a $\mathsf{NP}$-complete problem $A$ (for example $\texttt{SAT}$) and a $\mathsf{NEXP}$-complete problem $B$ (for example $\texttt{SUCCINCT HAMILTONIAN PATH}$), then $A\leqslant_m^p B$, $A\in \mathsf{NP}$ and $B\notin \mathsf{NP}$.

Note that more simply, you can consider $A=\emptyset$ and $B$ any problem not in $\mathsf{NP}$ and the result still holds.

  • $\begingroup$ Ah direct counter example and so the statement is false then. Could you explain what $A = \emptyset$ means? Is A literally an empty set or is this some kind of notation for A being the halting problem (I am simply not familiar with the interpretation of $\emptyset$ when discussing $P$ and $NP$) $\endgroup$ Commented Dec 10, 2022 at 1:18
  • 1
    $\begingroup$ A problem is a set. So $A = \emptyset$ means the empty problem. It can be interpreted as "the problem where the answer is always FALSE". $\endgroup$
    – Nathaniel
    Commented Dec 10, 2022 at 10:28

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