P = Np Conjecture (反)

2008-03-01 1:23 pm
六大未解決難題, 第三彈 Part II~


今次的 Paper 嘗試証明 SUBSET-SUM Problem 係冇 Polynomial Time, 從而証明 P =/= NP
http://arxiv.org/PS_cache/cs/pdf/0607/0607093v2.pdf
問: 証明那裏出錯了?
(呢份 Paper 只有兩頁... 比佢証明到, 唔單止數學界, 電腦界都要摺埋...)
參考:
http://en.wikipedia.org/wiki/Subset_sum_problem
更新1:

It is known that every pair of subsets must be compared. For example, take S = {1,2,4,8,16,... 2^n} Then each subsets will give you a different sum. So looking into all subsets is necessary.

更新2:

What I am thinking is his arguments on "imaginery processor", on the point about the running time to create such processors.

更新3:

Another note: Because the algorithm required is deterministic, the running time should not depends on the input b. That is, may be for some b, you can prune away a large number of subsets, it is possible that for some other b, you cannot throw away anything at all.

更新4:

Take my example above: since each subsets has different sum, reduction for one b probably cannot work for other b

更新5:

That is my point: the running time should be independent of b. So if I take the b large enough, you can't prune anything, and your running time will be slow.

回答 (1)

2008-03-02 9:03 am
✔ 最佳答案
The flaw of this paper is exactly what the paper emphasis, "the problem has a special mathematical structure". The proof assumes every pair of subsets has to be compared and used sorting to optimize number of comparisons. However, not every pair of subsets has to be compared. A large set of subsets can be removed when one comparison fails. [One can prune by odd/even, >0 or <0, or even better, not using that comparison method at all]
In particular, one cannot assert the lower bound of the problem (i.e. the least amount of time needed to solve the problem) by unnecessary constrainting the way to compute it.

2008-03-02 11:29:43 補充:
It is known that every pair of subsets must be compared.
For example, take S = {1,2,4,8,16,... 2^n}
Then each subsets will give you a different sum. So looking into all subsets is necessary.

2008-03-02 11:29:54 補充:
That is wrong. Different sums doesn't mean I have to compare.
The subsets that can be pruned does not mean they have the same sum, the subsets that can be pruned are those known to fail.

2008-03-02 11:29:59 補充:
For example, if there is a subset [of all +ve numbers] has a sum larger than b, then all the supersets of that subsets can be pruned. if there is no such subset (i.e. the sum of all numbers are smaller than B), the algorithm can return false.

2008-03-11 20:21:32 補充:
Let me be more precise and point to the paper. "Explanation: As we discussed earlier, in order to search a set of size N in a more efficient way than a brute-force search by a single computer processor, it is necessary for many processors to search the set in parallel, ... ".

2008-03-11 20:21:40 補充:
There is no need to search that set of all subsets, these subsets has some relationship within themselves that can help solve the problem.
參考: 從不抄襲。


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