Giảm Karp có giống với Giảm Levin không

Định nghĩa: Giảm Karp

Một ngôn ngữ $A$ là Karp rút gọn về một ngôn ngữ $B$ nếu có một thời gian đa thức chức năng tính toán $f:\{0,1\}^*\rightarrow\{0,1\}^*$ như vậy mà cho mỗi $x$ , $x\in A$ khi và chỉ khi $f(x)\in B$ .

Định nghĩa: Giảm Levin

Một vấn đề tìm kiếm $V_A$ có thể giảm Levin thành một vấn đề tìm kiếm $V_B$ nếu có hàm thời gian đa thức $f$ mà Karp giảm $L(V_A)$ thành $L(V_B)$ và có các hàm tính toán thời gian đa thức $g$ và $h$ sao cho

$\langle x, y \rangle \in V_A \implies \langle f(x), g(x,y) \rangle \in V_B$ ,
$\langle f(x), z \rangle \in V_B \implies \langle x, h(x,z) \rangle \in V_A$

Là những mức giảm tương đương?

Tôi nghĩ rằng hai định nghĩa là tương đương. Đối với bất kỳ hai $\mathsf{NP}$ ngôn ngữ $A$ và $B$ , nếu $A$ là Karp rút gọn về $B$ , sau đó $A$ là Levin rút gọn về $B$ .

Đây là bằng chứng của tôi:

Hãy và là trường hợp độc đoán của khi là của . Giả sử và là cán bộ kiểm tra của và . Hãy để và được giấy chứng nhận độc đoán của và theo . Hãy là của theo . $x$ $\overline{x}$ $A$ $x'$ $B$ $V_A$ $V_B$ $A$ $B$ $y$ $\overline{y}$ $x$ $\overline{x}$ $V_A$ $z$ $x'$ $V_B$

Xây dựng thẩm tra mới và với giấy chứng nhận mới và : $V'_A$ $V'_B$ $y'$ $z'$

$V'_A(x,y'):$

: Nếu , từ chối. Nếu đầu ra . $y'=\langle 0,\overline{x},\overline{y}\rangle$ $f(x)\ne f(\overline{x})$ $V_A(\overline{x},\overline{y})$
: Output . $y'=\langle 1,z\rangle$ $V_B(f(x),z)$

$V'_B(x',z'):$

: Output . $z'=\langle 0,z\rangle$ $V_B(x',z)$
: Nếu , từ chối. Nếu không thì đầu ra . $z'=\langle 1,x,y\rangle$ $x'\ne f(x)$ $V_A(x,y)$

Các hàm tính toán thời gian đa thức và được định nghĩa như sau: $g$ $h$

$g(x,y')$

$y'=\langle 0,\overline{x},\overline{y}\rangle$ : Output $\langle 1,\overline{x},\overline{y}\rangle$ .
$y'=\langle 1,z\rangle$ : Output $\langle 0,z\rangle$ .

$h(x',z')$

$z'=\langle 0,z\rangle$ : Output $\langle 1,z\rangle$ .
$z'=\langle 1,x,y\rangle$ : Output $\langle 0,x,y\rangle$ .

Let $Y_x$ be the set of all certificates of $x$ according to $V_A$ and $Z_{x'}$ be the set of all certificates of $x'$ according to $V_B$ . Then the set of all certificates of $x$ according to $V'_A$ is $0\overline{x}Y_\overline{x}+1Z_{f(x)}$ such that $f(x)=f(\overline{x})$ , and the set of all certificates of $x'$ according to $V'_B$ is $0Z_{x'}+1\overline{x}Y_\overline{x}$ such that $x'=f(\overline{x})$ .

(This is derived from the accepting language of $V'_A$ and $V'_B$ .)

Now let $x'=f(x)$ , the rest part is easy to check.

complexity-theory reductions check-my-proof

— c c
nguồn

Before proving your claim, you should define what it means by a language being Levin reducible to another.

— Tsuyoshi Ito

Câu trả lời:

No. First note that Levin reduction only makes sense with respect to classes which certificate has a meaning, e.g. $\mathsf{NP}$ while Karp reduction is general. The word "certificate" for a problem makes sense only when a verifier is fixed. Levin's reduction assumes that the verifiers are fixed. You cannot change the verifiers. (In the following I assume that certificate verifiers are fixed as required by definition of Levin reduction.)

Second, Levin reduction means that one can find the certificate for one from the certificate for the other. It is true that the well-known Karp reductions between natural problems turn out to be Levin reduction but this does not need to be true in general.

If we can reduce instances of a problem $A$ to problem $B$ it doesn't mean we have a way of computing a certificate for one from a certificate for the other.

For this to be true we need the fact that a certificate search problem corresponding to the decision problem is polynomial time reducible to the decision problem. This is true for $\mathsf{NP\text{-}complete}$ problems but not known to be true generally even for $\mathsf{NP}$ problems.

— Kaveh
nguồn

I agree on your first point that Karp reduction is more general than Karp reduction. According to it, I think my problem should be modified as "Let

L1 $L_1$ and

L2 $L_2$ be two languages in

NP $NP$ . If

L1 $L_1$ is Karp reducible to

L2 $L_2$ , then

L1 $L_1$ is Levin reducible to

L2 $L_2$ ." However, I don't agree on your other comments.

— c c

In my prove, first let

L1 $L_1$ and

$L_2$ be arbitrary such two languages. Since they are in

$NP$ , there are P TM

$M_1$ and

$M_2$ . For every instances

$x\in L_1$ , the set of all certificates are

$Y_x$ for

$TM_1$ . Similarly, we define

$Z_{x'}$ for

$x'\in L_2$ . Since

$L_1$ is Karp reducible to

$L_2$ , there is such

$f$ as defined.

— c c

Now, we constructed new

$M_1'$ and

$M_2'$ . For every instance

$x\in L_1$ , the new set of all certificates are

$0Y_x+1Z_{f(x)}$ , while for every instance

$f(x)\in L_2$ , the new set of all certificates are

$0Z_{f(x)}+1xY_x$ .(Here we use regular expressions) These are legal and all certificates for

$M_1'$ and

$M_2'$ . By the way, there is P functions

$g$ and

$h$ as defined transform all possible certificate from one problem to the other.

— c c

ps: We don't need to give a transformation from

$x'\in L_2$ where there is no

$x\in L_1$ such that

$x'=f(x)$ , since Karp and Levin reductions are both many to one mappings. I think this can answer the second last paragraph.

— c c

@cc, it seems that you still think that you can change the verifiers, you cannot, the definition of Levin reduction is for search problems, i.e. the verifiers are fixed.

— Kaveh

A quick counterexample for your proof: suppose that $x_1, x_2 \in L_1$ , $f(x_1) = f(x_2) \in L_2$ , and $w$ is a valid certificate for $x_1$ but not for $x_2$

$M_1'(x_1,\langle 0,w \rangle)= M_1(x_1,w)=1$

$M_1'(x_2,\langle 0,w \rangle)= M_1(x_2,w)=0$

By definition $g(x_1,\langle 0,w \rangle) = \langle 1,x_1,w \rangle$

$f(x_1)=f(x_2)$ so $M'_2(f(x_2),\langle 1,x_1, w \rangle)) = M_1(x_1,w) = 1$ so $\langle 1,x_1, w \rangle$ is a valid certificate for $f(x_2)$ but

$h(f(x_2), \langle 1,x_1, w \rangle) = \langle 0, w \rangle$ which is not a valid certificate for $x_2$

— Vor
nguồn

Thanks very much for pointing out the counterexample. I have modified the construction and I think it works now. Could you please have a look at it?

— c c