# Henc e dou ble sat is in n p ii dou ble sat is nphard

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Unformatted text preview: tanc e of LP AT H ” If ⟨G; a; b⟩ 2 U H AM P AT H , then G has a Hamiltonian path of length k from a to b . That is , ⟨G; a; b; k⟩ 2 LP AT H . On the other hand, if ⟨G; a; b; k⟩ 2 LP AT H , then G mus t c ontain a s imple path of length k from a to b . Howev er, s inc e G has only k nodes , the path mus t be Hamiltonian. There, ⟨G; a; b⟩ 2 U H AM P AT H : Henc e, U H AM P AT H Ôp LP AT H : Ac c ording to the abov e, LP AT H is NP­Complete. 7.21 i.) DOU BLE À SAT 2 N P On input of a Boolean formula, an NTM c ould gues s two different Boolean as s ignments nondeterminis tic ally and ac c ept if both of the as s ignments s atis fy the formula. Henc e, DOU BLE À SAT is in N P : ii.) DOU BLE À SAT is NP­hard SAT c an be reduc ed to DOU BLE À SAT by c ons truc ting a TM F that c omputes the reduc tion f as follows . F = ”On input of a Boolean formula (¶) with v ariables x1 ; x2 ; :::; xk 1. Let ¶0 = ¶ ^ (x _ x) by introduc ing a new v ariable x 2. Output (¶0) with v ariables x1 ; x2 ; :::; xk; x ” If ¶ 2 SAT , its s atis fy ing as s ignment plus x = true or x = true c an s erv e as the two s atis fy ing as s ignments of ¶0: On the other hand, if ¶0 2 DOU BLE À SAT is s atis fiable, ¶ c an be s atis fied by the as s ignment where the new v ariables x and x are remov ed. Therefore, DOU BLE À SAT is NP­hard. Ac c ording to i. and ii., DOU BLE À SAT is NP­Complete. 7.24 a.) In a c laus e of an = as s ignment, there is one literal s et true or two literals s et true. Thus , the = negation of a the c laus e will s till induc e a c laus e with one literal s et true or two literals s et true. Sinc e a negated c laus e is als o a c laus e of an = as s ignment, the negation of an = as s ignment is = = too an = as s ignment. = b.) Let ¶ be a 3c nf­formula of an ins tanc e of 3 SAT and ¶0 be a 3c nf­formula of an ins tanc e of = SAT : The reduc tion of replac ing a c laus e ci (y1 _ y2 _ y3 ) of ¶ with two c laus es = (y1 _ y2 _ zi) ^ (zi_ y3 _ b) of ¶0 c an be c omputed in poly nomial time for s ure. Nex t, we s how ¶ is s atis fiable iff ¶0 has the c orres ponding = as s ignment. = If ¶ is s atis fiable, either y1 = ture and y2 = ture , or only y3 = true . In the former c as e, s et zi fa...
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## This note was uploaded on 10/26/2013 for the course CS 22C:135 taught by Professor Zhang during the Spring '13 term at University of Iowa.

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