Given a list $L$ of sequences of the first $n+1$ natural numbers, how well can we approximate the shortest common supersequence of all sequences in $L$? The paper here shows that if $n$ is not restricted there is no constant factor polynomial time approximation. On the Approximation of Shortest Common Supersequences and Longest Common Subsequences

Does this remain true if $n$ is bounded or small say sublinear in $L$?

  • $\begingroup$ Related: cstheory.stackexchange.com/q/54062/8237 $\endgroup$
    – Neal Young
    Mar 22 at 0:38
  • 1
    $\begingroup$ @NealYoung Mapping doesn't work because we're looking at subseqences not substrings. If we try to map the set of charactersonto $k$ ary strings then choosing a minimum set i(0),i(1),.., i(k) of the first n+1 natural numbers such that b_i(0),b_i(1),.., b_i(k) contains each of 0,1,2,..,k then (b_i(0) b_i(1) .., b_i(k) )^* contains a supersequence of any sequence $\endgroup$
    – Hao S
    Mar 22 at 1:24
  • $\begingroup$ @NealYoung I'm not looking for a PTAS there are two "constants" in question the approximation ratio and the alphabet size. My question is there any $b \geq 0$ such that for any constant $c \geq 2 $ the SCS problem on an alphabet of size c admits a $b$-approximation? $\endgroup$
    – Hao S
    Mar 28 at 19:57
  • $\begingroup$ Actually it's "is there a constant b such that for ANY constant c≥2 , the c -supersequence problem is NP-hard to approximate within b? $\endgroup$
    – Hao S
    Mar 28 at 23:13
  • 1
    $\begingroup$ Hao, and I assume by "for ANY constant c" you mean "for ALL constants c", not "for SOME constant c." So, you are asking "does there exist a constant b such that, for every fixed c, b-approximating the c-supersequence problem (with running time polynomial in the input size, but depending arbitrarily on c and b) is NP-hard?" $\endgroup$
    – Neal Young
    Mar 29 at 0:47

1 Answer 1


Does this remain true if $n$ is bounded?

No. When $n=O(1)$, there is a constant-factor approximation algorithm. See Lemma 1 below.

Does this remain true if $n$ is sublinear in $|L|$?

Yes. For every constant $\epsilon>0$, the problem remains just as hard to approximate even when $n\le |L|^{\epsilon}$. See Lemma 2 below.

Lemma 1. There is a constant-factor approximation algorithm for the case when $n=O(1)$.

Lemma 2. For any $\epsilon>0$, there exists a $\delta>0$ such that, if the problem (restricted to $n\le |L|^{\epsilon}$) has a polynomial-time approximation algorithm with performance ratio $n^\delta$, then P=NP.

Proof of Lemma 1. Just return the sequence $$(1, 2, \ldots, n+1)^\ell$$ where $\ell=\max\{|\sigma| : \sigma \in L\}$. This sequence has length $(n+1)\ell$, while $\ell$ is a lower bound on the optimum, so this is an $(n+1)$-approximation algorithm. $~~\Box$

Proof of Lemma 2. Fix any $\epsilon>0$. We point out that the general problem reduces in polynomial time, preserving approximate solutions, to the restricted problem (requiring $n \le |L|^\epsilon$).

Given an instance $(L, n)$ of the general problem, the reduction produces the instance $(L_\epsilon, n)$ where $L_\epsilon$ is obtained from $L$ by adding $n^{1/\epsilon}$ empty sequences, so that $$|L_\epsilon| \ge n^{1/\epsilon}, \text{ that is } n\le |L_\epsilon|^\epsilon,$$ so that $(L_\epsilon, n)$ is indeed an instance of the restricted problem.

We assume WLOG here that every symbol in $[n+1]$ occurs in some string in $L$, so $n$ is at most the input size $\sum_{\sigma\in L} |\sigma|$, so the reduction takes polynomial time.

By the result cited by OP, there is a $\delta>0$ such that there is no poly-time $n^{\delta}$-approximation algorithm for the general problem unless P=NP. By the above reduction (which blows up the input size by at most a polynomial) the same is true for the restricted problem. $~~~~\Box$

  • $\begingroup$ I meant a constant independent of n. $\endgroup$
    – Hao S
    Mar 25 at 20:55
  • $\begingroup$ Which constant do you mean should be independent of $n$? And what does "independent of $n$" mean if we are restricting $n$ to be $O(1)$? $\endgroup$
    – Neal Young
    Mar 25 at 21:04
  • $\begingroup$ Is there a constant c such that for any n, given a list $L$ of sequences of the first $n+1$ natural numbers, we can approximate the shortest common supersequence of all sequences in $L$? within factor c in time polynomial in |L|? In other words first fix the the approximation ratio then fix n. $\endgroup$
    – Hao S
    Mar 25 at 21:32
  • $\begingroup$ I don't see how $n$ is "fixed" as you describe it. Could you make all the quantifiers you have in mind explicit? $\endgroup$
    – Neal Young
    Mar 25 at 22:11
  • $\begingroup$ Do you understand the statement before "in other words" $\endgroup$
    – Hao S
    Mar 25 at 23:27

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