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Solutions for Homework 2 MA 522 Fall 2008 1. (a) Show that an otherwise polynomial-time algorithm that makes at most a constant number of calls to polynomial-time subroutines runs in polynomial time; Proof: Assume that for 1 ≤ i ≤ c our algorithm calls a subroutine Si with input xi of binary length mi . Assume that Si performs at most nei binary operations on inputs of length n, where ei doesn’t depend on n. Let ne0 is an upper bound for the running time for our algorithm on input of length n – without the subroutine calls. Then ne0 + me1 + . . . + mec 1 c is the total running time of the algorithm (with subroutine calls) on an input of length n. Here c, e0 , . . . , ec do not depend on n. By induction we will prove that the input size mi of Si is at most nfi for fi ≤ i−1 ej – not depending on n. For i = 1 we have f1 ≤ e0 , j=0 which proves the base case. Assume that the mi−1 ≤ nfi−1 . Then the output size of Si−1 is at most (nfi−1 )ei−1 , therefore the input size of Si is at most (nfi−1 )ei−1 = nfi−1 (ei−1 ) . (We can assume without loss of generality that the output of Si−1 is the input of Si .) Therefore, fi ≤ i−1 ej , which proves the inductive step. Thus we have that the total running time j=0 is at most e1 ec c c ne0 + nf1 + . . . + nfc ∈ O(n(maxj=0 ej ) ) and the exponent of the right hand side does not depend on n. (b) Show that a polynomial number of calls to polynomial time subroutines may result in an exponential-time algorithm. Proof: Here is an example for such algorithm: Each subroutine Si has running time at most ne for inputs of size n, but the input size of Si is mi := 2i−1 n. For example Si squares its input x in linear time. Then Sn has input size 2n−1 n, so its running time is linear in 2n−1 n, which is not polynomial time. 2. Let FACTORING = {(n, k) : n has some factor d with 1 < d < k}. (a) Show that FACTORING ∈ NP; Proof: The witness is (n, k, d), and the verifying algorithm checks whether 1 < d < k and d|n, which is clearly polynomial time. (b) Show that FACTORING ∈ co-NP (using that PRIMES ∈ NP). Proof: Witness: (n, k, p1 , . . . , pr , wit(p1 ), . . . , wit(pr )) where (pi , wit(pi )) is the NP-witness for pi ∈ PRIMES. The verifying algorithm checks the following things: 1 r 1. n = i=1 pi 2. 1 < k < pi for all 1 ≤ i ≤ r 3. verify that (pi , wit(pi )) satisﬁes the PRIMES certiﬁcate Note that the size of the certiﬁcate is polynomial in log(n), since n has at most log(n) prime factors of size at most log(n), and for each prime of size at most log(n), its PRIMES witness has also polynomial size in log(n). 3. (a) Prove that if NP = co-NP, then P = NP. Proof: Assume that P=NP. Then the polynomial time algorithms for an NP language L can be used to decide whether x ∈ L, thus P=co-NP, so NP=co-NP, a contradiction. (b) Prove that if FACTORING is NP-complete then NP = co-NP. Proof: If FACTORING is NP-complete then for all L ∈ NP L ≤P FACTORING. Since FACTORING is in co-NP, it implies that all L ∈ NP L ∈ co-NP, i.e.NP ⊆ co-NP. Now ¯ let L ∈ co-NP. Then L ∈ NP ⊆ co-NP, thus L ∈ NP, i.e. co-NP ⊆ NP. This proves that NP=co-NP. 4. The subgraph-isomorphism problem takes two graphs G1 and G2 and asks whether G1 is isomorphic to a subgraph of G2 . (Two graphs are isomorphic, if there is a permutation of the verteces which transform one graph into the other, preserving the edges). Prove that the subgraph isomorphism problem is NP-complete, using NP-complete problems discussed in class. Proof: First we prove that the subgraph-isomorphism problem is in NP. The certiﬁcate is (G1 = (V1 , E1 ), G2 = (V2 , E2 ), φ : V1 → V2 ). The verifying algorithm checks if φ isa one- to-one function, and for all u, v ∈ V1 whether (u, v) ∈ E1 if and only if (φ(u), φ(v)) ∈ E2 . Secondly, we prove that CLIQUE ≤P SUBGRAPH ISOMORPHISM. Let (G = (V, E), k) be an input instance for CLIQUE. Deﬁne G1 to be the complete graph on k vertices, and G2 to be the grapgh G. Then (G1 , G2 ) ∈ SUBGRAPH ISOMORPHISM if and only if (G, k) ∈ CLIQUE. 2

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