Theoretical Computer Science ELSEVIER’

Theoretical

Computer

Science 207 (1998)

3 19-328

On common information AI-LA. Muchnik *

Abstract We present a complexity-theoretic proof of the result of P.Ggcs and J.KGrner on the existence of a pair of words whose common information can not be materialized. Our method is easier than G&s and Kijrner’s method and gives a possibility to get some generalization of the result. Besides, we show that there are many enough pairs of words with this property. @ 199X Elsevier Science B.V. All rights reserved K~~NYHY&: Kolmogorov entropy; Algorithmic information theory; Common information of two words

1. Introduction The results of the present paper were announced In paper [2] Kolmogorov

gave the definition

and the definition of the quantity of information it was proved that the quantity of information term of O(logK(x)

+ logK(y))

(all logarithms

in [3].

of the entropy

K(x) of finite object x

Z(x : y) about y contained is commutative to within

in x. In [4] an additive

in our paper have base two). In that

paper it was also proved that

IZ(x: Y>- (K(x) + K(y) - KC-% Y>>l= O(logK(x) + logK(y)). Because of commutativity of Z(x : y) we will call Z(x : y), Z(y : x) and K(x) + K(y) - K(x, y) the quantity of common information of x, y. My father, A.A. Muchnik, raised the following question: is there a word = such that K(z) = Z(z : x) = Z(z : y) = Z(x : y)? That is, can we materialize the common information? As Z(x : y) is commutative only to within an additive term of O(logK(x) + log K(y)) the exact formulation of this question is the following. The problem of A.A. Muchnik. Is the following assertion true?

* Fax: 7095 915-69-63; e-mail:

0304.3975/98/$19,00

[email protected]

@ 1998 -Elsevier

PfISO304-3975(98)00070-X

Science B.V. All rights reserved

An.A. Muchnikl Theoretical Computer Science 207 (1998) 319-328

320

For some constant

K(z)6Z(x

c for all binary words x, y there is a binary word z such that

: y) + c(logK(x)

+ logK(y))

+ c,

Z(z : x) 31(x

: y) - c(logK(x)

+ logK(y))

- c,

I(z : y)>I(x

: y) - c(logK(x)

+ logK(y))

-c.

In paper [I] Gacs and Korner gave a negative

(1)

solution

to this problem.

probabilistic methods. In the present paper we present a complexity-theoretic to the problem of A.A. Muchnik, which is easier than Gacs and Korner’s

They used solution solution.

We are interested also in the following generalization of the question. Let us fix the parameters m, n, a, b, i E N such that a a + i,

n>,b+i. Theorem 2. There is u constant

c E N such that thr fbllobving

n, u, b, i such that m3i,

m>a,

nbi,

n3 b there ure x, y

holdx

such

For rrer~ m,

that K(X) ==(,m.

I =(. n, f(x : y) - il6 5(log m + log n) + c, C&(X, JJ)> max{a, b, min{a + n - i, b + m--i,a+b}}-5(logm+logn)-c. Proof. Let m, n, a, b, i satisfy the inequalities m 3 i, n > i, m > a, n 3 b. Let us denote k=min{a+b,a+n-i,b+m-i}. It is easy to see that C&x, y) 3 a - 5(log m + log n) - c and Cu,(x, y) 3 b - 5(log m + logn) appropriate c.

- c. So we have to prove that C&x, y) 3 k - c’ for

We will define two words x and y satisfying the conditions of the theorem. The words x, y must be such that there is no z such that K(z) < k - c, Z(z : ~)>a, I(: v)>,b. The last two inequalities means that K(xlz)