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Prove that NP is closed under karp reduction?

Space(n) not closed under Karp reductions - what about NTime(n)?Class P is closed under rotation?Prove or disprove that $NL$ is closed under polynomial many-one reductions$mathbfNC_2$ is closed under log-space reductionOn Karp reductionwhen can I know if a class (complexity) is closed under reduction (cook/karp)Check if class $PSPACE$ is closed under polyonomially space reductionIs NPSPACE also closed under polynomial-time reduction and under log-space reduction?Prove PSPACE is closed under complement?Prove PSPACE is closed under union?

1

$begingroup$

A complexity class $mathbbC$ is said to be closed under a reduction if:

$A$ reduces to $B$ and $B in mathbbC$ $implies$ $A in mathbbC$

How would you go about proving this if $mathbbC = NP$ and the reduction to be the karp reduction? i.e.

Prove that if $A$ karp reduces to $B$ and $B in NP$ $implies$ $A in NP$

complexity-theory

share|cite|improve this question

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Try using the definitions.
  $endgroup$
  – Yuval Filmus
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @YuvalFilmus thanks for the advice, this helped me figure it out!
  $endgroup$
  – Ankit Bahl
  3 hours ago
  

  
  
  
  

add a comment | 

1

$begingroup$

A complexity class $mathbbC$ is said to be closed under a reduction if:

$A$ reduces to $B$ and $B in mathbbC$ $implies$ $A in mathbbC$

How would you go about proving this if $mathbbC = NP$ and the reduction to be the karp reduction? i.e.

Prove that if $A$ karp reduces to $B$ and $B in NP$ $implies$ $A in NP$

complexity-theory

share|cite|improve this question

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Try using the definitions.
  $endgroup$
  – Yuval Filmus
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @YuvalFilmus thanks for the advice, this helped me figure it out!
  $endgroup$
  – Ankit Bahl
  3 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

A complexity class $mathbbC$ is said to be closed under a reduction if:

$A$ reduces to $B$ and $B in mathbbC$ $implies$ $A in mathbbC$

How would you go about proving this if $mathbbC = NP$ and the reduction to be the karp reduction? i.e.

Prove that if $A$ karp reduces to $B$ and $B in NP$ $implies$ $A in NP$

complexity-theory

share|cite|improve this question

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|cite|improve this question

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|cite|improve this question

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 4 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 4 hours ago

Ankit BahlAnkit Bahl

262

1 Answer
1

active

oldest

votes

2

$begingroup$

I was able to figure it out. In case anyone was wondering:

$B in NP$ means that there exists a non-deterministic polynomial time algorithm for $B$. Let's call that $b(i)$, where i is the input to $B$.

$A$ karp reducing to $B implies$ that there exists a function $m$ such that $m$ can take an input $i$ to $A$ and map it to some input $m(i)$ for $B$, and if an instance of $i$ is true for $A$ then $m(i)$ is true for B (and vice versa),

Therefore, an algorithm for $A$ can be made as follows:

$A (i)$

1. Take input $i$ and apply $m$ to yield $m(i)$
   


2. Apply $b$ with input $m(i)$
   

This yields an output for $A$. Since both $m$ and $b$ are non-deterministic polynomial time, this algorithm is non-deterministic polynomial time. Therefore $A$ must be in NP.

share|cite|improve this answer

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

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2

$begingroup$

I was able to figure it out. In case anyone was wondering:

$B in NP$ means that there exists a non-deterministic polynomial time algorithm for $B$. Let's call that $b(i)$, where i is the input to $B$.

$A$ karp reducing to $B implies$ that there exists a function $m$ such that $m$ can take an input $i$ to $A$ and map it to some input $m(i)$ for $B$, and if an instance of $i$ is true for $A$ then $m(i)$ is true for B (and vice versa),

Therefore, an algorithm for $A$ can be made as follows:

$A (i)$

1. Take input $i$ and apply $m$ to yield $m(i)$
   


2. Apply $b$ with input $m(i)$
   

This yields an output for $A$. Since both $m$ and $b$ are non-deterministic polynomial time, this algorithm is non-deterministic polynomial time. Therefore $A$ must be in NP.

share|cite|improve this answer

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

2

$begingroup$

I was able to figure it out. In case anyone was wondering:

$B in NP$ means that there exists a non-deterministic polynomial time algorithm for $B$. Let's call that $b(i)$, where i is the input to $B$.

$A$ karp reducing to $B implies$ that there exists a function $m$ such that $m$ can take an input $i$ to $A$ and map it to some input $m(i)$ for $B$, and if an instance of $i$ is true for $A$ then $m(i)$ is true for B (and vice versa),

Therefore, an algorithm for $A$ can be made as follows:

$A (i)$

1. Take input $i$ and apply $m$ to yield $m(i)$
   


2. Apply $b$ with input $m(i)$
   

This yields an output for $A$. Since both $m$ and $b$ are non-deterministic polynomial time, this algorithm is non-deterministic polynomial time. Therefore $A$ must be in NP.

share|cite|improve this answer

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

$begingroup$

I was able to figure it out. In case anyone was wondering:

$B in NP$ means that there exists a non-deterministic polynomial time algorithm for $B$. Let's call that $b(i)$, where i is the input to $B$.

$A$ karp reducing to $B implies$ that there exists a function $m$ such that $m$ can take an input $i$ to $A$ and map it to some input $m(i)$ for $B$, and if an instance of $i$ is true for $A$ then $m(i)$ is true for B (and vice versa),

Therefore, an algorithm for $A$ can be made as follows:

$A (i)$

1. Take input $i$ and apply $m$ to yield $m(i)$
   


2. Apply $b$ with input $m(i)$
   

This yields an output for $A$. Since both $m$ and $b$ are non-deterministic polynomial time, this algorithm is non-deterministic polynomial time. Therefore $A$ must be in NP.

share|cite|improve this answer

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|cite|improve this answer

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 3 hours ago

Ankit BahlAnkit Bahl

262

New contributor

Ankit Bahl is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 3 hours ago

Ankit BahlAnkit Bahl

262