sharp bounds on davenport-schinzel sequences of every order seth pettie university of michigan
TRANSCRIPT
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Sharp Bounds onDavenport-Schinzel Sequences
of Every Order
Seth PettieUniversity of Michigan
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Lower Envelopes• Given n continuous functions defined over a common interval.• Each pair of functions intersect at most s times.• The lower envelope is the pointwise minimum of the functions.
An order-s Davenport-Schinzel sequence contains no subsequences isomorphic to a…b…a…b… (length s+2).
(s=4 in this case)
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Lower Envelopes• Given n continuous functions, each defined over some interval.• Each pair of functions intersect at most s times.
Contains no subsequences isomorphic to a…b…a…b… (length s+4)
(s=1 in this case)
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The extremal function ls
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Numerous applications of DS sequences
• Geometric arrangements• Geometric containment probs.• Dynamic geometric algs.• Motion planning probs.• Kinetic data structures• Self-adjusting data structs.• …
… Better understanding of ls(n)
Better understanding of (geometric) algorithmsand data structures, arrangements, etc.
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Davenport-Schinzel 1965, Davenport 1970
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Szemerédi 1973
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Hart-Sharir 1986
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Sharir 1987, Sharir 1988,
Agarwal-Sharir-Shor 1989
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Nivasch 2009 (& Klazar 1999)
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New Results
• The moral?– Order 5 is perceptibly different from order 4.– Even and odd s are imperceptibly different at order s≥6
but they require rather different analyses.– It is impossible to give improved bounds, expressed in
terms of “a(n).”
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A “block” is a sequence of distinct symbols.
(Hart-Sharir’86, Sharir’88: ls(n) is not much larger than ls(n,n).)
The goal: Find a good recurrence relation for ls(n, m).
This only requires reasoning about the structure of DS sequences. (The inverse-Ackermann function only comes into play in the analysis of the recurrences.)
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Sequence DecompositionFrom [Agarwal-Sharir-Shor’89]
• S is an order-s, m-block sequence.
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Sequence Decomposition
• S is an order-s, m-block sequence.• Partition S into intervals of blocks.– Local symbols are contained in one interval
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• S is an order-s, m-block sequence.• Partition S into intervals of blocks.– Local symbols are contained in one interval– Global symbols appear in multiple intervals.
Sequence Decomposition
firstoccurrences
lastoccurrences
middleoccurrences
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• S is an order-s, m-block sequence.• Partition S into intervals of blocks.• Contract each interval into a single block, forms
(Keep one copy of each global symbol in the interval)
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Symbol occurrences fall in four categories:–First global–Last global–Middle global–Local
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Order s=4 throughout this example. (No ababab.)
How many “first” occurrences are there in S?
block contains firstoccurrence of
both a & b
at least one a and bfollowing the block
Must be an order-(s-1)DS sequence. (No ababa.)
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Order s=4 throughout this example. (No ababab.)
How many “middle” occurrences are there in S?
at least one a and bpreceding the block
a & b are “middle”occurrences in
this block
at least one a and bfollowing the block
Must be an order-(s-2)DS sequence. (No abab.)
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A recurrence for ls [Agarwal-Sharir-Shor’89, Nivasch’09]
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The Derivation Tree
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The Derivation Tree
Nodes blocks encountered⟷in the sequence decomposition
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The Derivation Tree
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The Derivation Tree
height≈ a(n,m)
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The Derivation Tree for Order-3 DS seqs.
What it should look like in an idealized form:
if tree has height a(n,m) about 2a(n,m) leaves.
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The Derivation Tree for Order-4 DS seqs.
What it should look like in an idealized form:
if tree has height a(n,m) Q(2a(n,m)) leaves.
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Why is it difficult to realize such trees at the odd orders (s ≥ 5)?
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Nesting and the Odd Orders
a and b nested in this block
a and b interleaved in this block
order-2 (no abab)
order-3 (no ababa)
Order 4:(No ababab)
Order 5:(No abababa)
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Nesting and the Odd Orders
a and b nested in this block
order-(s-3)
Order s=5:(No abababa)
Suppose interleaving were outlawed…
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• If you want to prove Nivasch’s bound on ls is tight:– Show how to construct sequences where interleaving is
the norm.
• If you want to prove it isn’t tight:– Prove interleaving is rare and that
any interleaved symbols can be segregated.
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Anatomy of a derivation tree
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Anatomy of a derivation tree
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Anatomy of a derivation tree
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Anatomy of a derivation tree
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• The Structural Lemma:
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• Implications of the Structural Lemma (for odd s):– Three distinct branching factors for wingtips, feathers,
and all other non-feathers:
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A recurrence for odd orders
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• Need to solve three interconnected recurrences:– ls(n, m) for even s [Nivasch’09]
– ls(n, m) for odd s.
– Fs(n, m) for odd s.
…not difficult — just a big proof by induction.
Only shows l5(n)=O(n(a(n))22a(n)) not O(na(n)2a(n)).
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Order-5 DS sequences are very tricky
l5(n) = W(na(n)2a(n)) construction requires several new ideas.
l5(n) = O(na(n)2a(n)) upper bound requires that we look at a two-layer derivation tree.
derivation tree fororder-5 DS sequence
derivation tree fororder-4 DS sequence.Leaves = quills in T|a.
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Open Problems
• Bounding ls(n) essentially a closed problem.
• Bounds on generalized DS sequences? (When the forbidden subsequence is not of the form ababa…)
• Realizing DS sequences by natural functions or natural geometric objects is still wide open.– Can ls(n) be realized by degree-s polynomials?
– Can ls+2(n) be realized by degree-s poly. segments?
–
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Proof of the Structural Lemma
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w.lo.g., root of T|b ancestral to root of T|a
w.lo.g., v is a dove in T|a
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if a & b are interleaved in v…
then all bs appear hereor here
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No, then v would be a dove feather in T|b
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then a & b are nested in v.
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Second case: all bs appear here
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No, then v would be a hawk feather in T|b
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Then this node wasn’t the first wing nodeancestor of v in T|a—contradiction.
a must appear in thisblock (not obvious).
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The structural lemma for order-5 DS sequences is similar but more involved.