## Almost linear upper bounds on the length of general Davenport-Schinzel sequences.(English)Zbl 0636.05004

(n,s)-Davenport-Schinzel sequences are sequences that are composed of n symbols, and are such that (i) no two adjacent elements are equal, and (ii) there does not exist a (not necessarily contiguous) alternating subsequence of length $$s+2$$ of the form a...b...a...b... for any two distinct symbols a and b. Davenport-Schinzel sequences arise in the computation and analysis of the lower or upper envelope of a set of functions, and are thus a powerful and versatile tool for a variety of problems in combinatorial and computational geometry. This paper establishes almost linear upper bounds on the maximum length $$\lambda_ s(n)$$ of (n,s) Davenport-Schinzel sequences. These bounds are of the form $$O(n\alpha (n)^{O(\alpha (n)^{s-3})})$$, and they generalize and extend the tight bound $$\Theta$$ (n$$\alpha$$ (n)) obtained by S. Hart and the author [ibid. 6, 151-177 (1986; see above, Zbl 0636.05003)] for the special case $$s=3$$ ($$\alpha$$ (n) is the extremely slowly growing functional inverse of Ackermann’s function), and also improve the upper bound O(n log *n) due to E. Szemeredi (1974).
The results of this paper have later been slightly improved by Agarwal, Sharir and Shor (1988), who have shown that $\lambda_{2s}(n)=O(n\cdot 2^{O(\alpha (n)^{s-1})}),\quad for\quad s\geq 2,\quad \lambda_{2s+1}(n)=O(n\cdot \alpha (n)^{O(\alpha (n)^{s-1})}),\quad for\quad s\geq 2.$ Moreover, these bounds are almost tight for even s, because $\lambda_{2s}(n)=\Omega (n\cdot 2^{\Omega (\alpha (n)^{s- 1})}),\quad for\quad s\geq 2.$
Reviewer: M.Sharir

### MSC:

 05A10 Factorials, binomial coefficients, combinatorial functions

### Keywords:

Davenport-Schinzel sequences; forbidden subsequences

Zbl 0636.05003
Full Text:

### References:

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