記集合A = {x:G(x)表示x滿足某種性質(zhì)},表達(dá)元素x與集合A的所屬關(guān)系,即“整體中個體”,而判斷x是否在A中,即判斷元素x是否具有性質(zhì)G(x)。
從這個理解出發(fā),SAT呈現(xiàn)出二個層次上的定義:
-判定問題:窮舉法判定SAT問題的給定實(shí)例x是否可滿足。這相當(dāng)于,窮舉法判定x的給定候選解y是否在R中。
-判斷問題:如果窮舉法可以判定SAT實(shí)例(R),那么窮舉法是否可以判定SAT問題(L)?
可見,“判定問題”面向“實(shí)例”(個體,R),而“判斷問題”面向“問題”(整體,L),所以NP的形式定義是基于對“判斷問題”的回答。實(shí)際上,我們也能看到,這個回答才是造成現(xiàn)有的NP定義的困難的根源。
對于“判定問題”,通過枚舉SAT實(shí)例x的所有候選解y,重復(fù)調(diào)用多項(xiàng)式時(shí)間驗(yàn)證解的圖靈機(jī)M,就可以在指數(shù)時(shí)間判定x是否可滿足,所以窮舉法對判定SAT實(shí)例x具有可計(jì)算性。
但要注意,這個意義上的“窮舉法”并沒有產(chǎn)生一個新的圖靈機(jī)與之對應(yīng),就是說,不過是重復(fù)調(diào)用多項(xiàng)式時(shí)間驗(yàn)證解的圖靈機(jī)M,因此這里的“指數(shù)時(shí)間”只是表達(dá)重復(fù)調(diào)用M,是一個由“人”暗中定義了的“指數(shù)時(shí)間復(fù)雜度”。所以,“判定問題”的本質(zhì)是“P問題”。
根據(jù)NP的形式定義,“語言L在NP中,當(dāng)且僅當(dāng)存在多項(xiàng)式可判定且多項(xiàng)式平衡的關(guān)系R,使得語言L = {x : (x,y)在R中存在y}”,就是說,“窮舉法”判定SAT實(shí)例與“窮舉法”判定SAT問題等價(jià),換句話說,在“判定問題”與“判斷問題”之間建立起了越界的等價(jià)關(guān)系!
這也正是NP的非形式定義1與定義2“等價(jià)”所表達(dá)的意義:“NP is the class of problems verifiable by TM in polynomial time”與“NP is the class of problems decidable by NDTM in polynomial time”等價(jià)。
所以,流行NP定義是對NP問題的“可計(jì)算性”的直接肯定,而非論證,實(shí)例與問題的關(guān)系從“整體中的個體”變成了“個體即整體”,“判斷問題”因此被取消了,從而失去了揭示NP本質(zhì)的可能性,暗含NP=P。下面我們從現(xiàn)有的“可計(jì)算性”觀念中追究更一般性的原因。
3,流行的“可計(jì)算性”所的隱含的矛盾:“圖靈機(jī)”與“可計(jì)算問題”
在現(xiàn)有的計(jì)算機(jī)理論中,“可計(jì)算問題”被解讀為,“可計(jì)算問題是由’停機(jī)’的圖靈機(jī)計(jì)算的問題”,圖靈機(jī)的“無限長的紙帶”被解讀為“無限的時(shí)空”,所以圖靈機(jī)的計(jì)算被解釋為“不計(jì)時(shí)空開銷”。這樣的“可計(jì)算問題”在“理論上”似乎是可計(jì)算的,但“物理上”卻不一定是可計(jì)算的。
“圖靈機(jī)模型使用無限長紙帶作為其無限內(nèi)存,有一個讀寫頭。最初,輸入字符串被放置紙帶上,紙帶的其他方格均為空白。機(jī)器將繼續(xù)計(jì)算,直到?jīng)Q定產(chǎn)生輸出為止。通過進(jìn)入的接受和拒絕狀態(tài)來判斷是否接受和拒絕輸入,如果圖靈機(jī)不進(jìn)入接受或拒絕狀態(tài),則將永遠(yuǎn)持續(xù)下去,永不停止。[10](Sipser書,Theorem 3.16)”
既然圖靈機(jī)的計(jì)算“不計(jì)時(shí)空開銷”,那么“窮舉法”計(jì)算NP問題的實(shí)例與計(jì)算NP問題(任意實(shí)例)就沒有區(qū)別,這就是“判定問題”與“判斷問題”之間越界的等價(jià)關(guān)系的來源!
現(xiàn)在讓我們追本溯源回到圖靈的可計(jì)算性理論,考察“NP問題是窮舉法可計(jì)算的”有效性。
四,圖靈的“可計(jì)算性”:“NP問題是窮舉法可計(jì)算的嗎?”
作為計(jì)算機(jī)理論的核心概念,“可計(jì)算性”表達(dá)了“算法”普遍性的解決問題的過程性能力,對這種過程性能力的考察被數(shù)學(xué)家隱含地表達(dá)出來,這就是著名的希爾伯特(David Hilbert 1862─1943)的Entscheidungsproblem:是否存在“通用過程”來判定任何可定義的數(shù)學(xué)問題可解。
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