# A max-even subset problem

I want to know if there is any polynomial algorithm for the problem, or any NP-completeness result.

• Given a set $S$ and $m$ subsets $C_1, \dots, C_m$ of $S$, we want to find a non-empty set $X\subseteq S$ such that $|X\cap C_i|$ is even for as many $i$ as possible.

We can see this problem in another way: given $m$ $\{0,1\}$-vectors of size $n$, find a vector $X$ of size $n$ that has even inner product with the greatest possible number of the vectors.

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– Raphael
Jun 22, 2016 at 14:39
• I guess you want to find $X$ as large as possible?
– Raphael
Jun 22, 2016 at 14:40
• i do a research on computer science and i reached to this problem and i think that it might be a famous problem, but i cant find this with search and i can't solve it myself, so i decided to post it here.no, X can be any non empty subset. the goal is to maximize the number of Ci's. Jun 22, 2016 at 18:54

Input: a $n \times n$ matrix $M$ of integers modulo 2
Goal: find a $n$-vector $x$ such that the Hamming weight of $Mx$ is as small as possible (with all arithmetic modulo 2)

This is the problem of finding a minimum-weight codeword for a linear code over $GF(2)$. This problem is NP-hard. See, e.g.,

Elwyn R. Berlekamp, Robert J. McEliece, and Henk C. A. van Tilborg. On the Inherent Intractability of Certain Coding Problems. IEEE Transactions on Information Theory 24 (1978), 384–386.

There are heuristics and algorithms that improve upon brute-force search, and algorithms that can work in polynomial time for certain special cases, but in the general case this is still hard if $n$ is large.

• thank you for answer. are you sure that this problem is exactly the problem that paper discusses about? i think that is another problem, wants to minimize the weight of x such that xA=0, for given A. is there any reduction ? Jun 23, 2016 at 8:01
• @MohammadJavadHajialikhani, I think it's equivalent, if we let $A$ be the parity check matrix of the code given by the generator matrix $M$. (I could be wrong.)
– D.W.
Jun 23, 2016 at 16:52