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214 Finding Paths in Tournaments
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596 {What is a Tournament?}
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630 Every pair has a joust.
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1014 Tournaments Arise Naturally in Different Situations
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1023 {Applicatins in Ordering Theory}
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1052 {Applications in Sociology}
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1081 {Applications in Structural Complexity Theory}
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2320 approximately their distance.
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2330 Standard Complexity Classes
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2563 Logspace Turing Machines Are Quite Powerful
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2572 {Deterministic logspace machines can compute}
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2821 \begin_layout Plain Layout
2825 \begin_layout Plain Layout
2829 \begin_layout Plain Layout
2840 \begin_layout Plain Layout
2844 \begin_layout Plain Layout
2854 operatorname{forest}}$}}
2857 \begin_layout Plain Layout
2861 \begin_layout Plain Layout
2865 \begin_layout Plain Layout
2869 \begin_layout Plain Layout
2880 \begin_layout Plain Layout
2884 \begin_layout Plain Layout
2903 \begin_layout Plain Layout
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2926 \begin_layout Plain Layout
2930 \begin_layout Plain Layout
2943 \begin_layout Plain Layout
2947 \begin_layout Plain Layout
2955 operatorname{forest}}$,}
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2964 \begin_layout Plain Layout
2972 operatorname{forest}}$,}
2977 \begin_layout Plain Layout
2981 \begin_layout Plain Layout
2989 operatorname{path}}$,}
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2998 \begin_layout Plain Layout
3006 operatorname{path}}$}}}}
3009 \begin_layout Plain Layout
3013 \begin_layout Plain Layout
3023 operatorname{tourn}}$}}
3026 \begin_layout Plain Layout
3030 \begin_layout Plain Layout
3043 \begin_layout Plain Layout
3047 \begin_layout Plain Layout
3055 operatorname{tourn}}$,}
3060 \begin_layout Plain Layout
3064 \begin_layout Plain Layout
3075 \begin_layout Plain Layout
3079 \begin_layout Plain Layout
3088 \begin_layout Plain Layout
3092 \begin_layout Plain Layout
3098 pgfsetdash{{1pt}}{0pt}
3104 operatorname{tourn}}$''}}
3107 \begin_layout Plain Layout
3111 \begin_layout Plain Layout
3123 \begin_layout BeginFrame
3124 The Circuit Complexity Classes AC
3125 \begin_inset Formula $^{0}$
3129 \begin_inset Formula $^{1}$
3133 \begin_inset Formula $^{2}$
3137 \begin_inset Newline newline
3140 Limit the Circuit Depth
3143 \begin_layout Standard
3147 \begin_layout Plain Layout
3156 \begin_layout Plain Layout
3160 \begin_layout Plain Layout
3172 \begin_layout Columns
3173 \begin_inset Argument
3176 \begin_layout Plain Layout
3186 \begin_layout Column
3194 \begin_layout Plain Layout
3202 \begin_inset Formula $\Class{AC}^{0}$
3209 \begin_layout Plain Layout
3220 \begin_layout Itemize
3221 \begin_inset Formula $O(1)$
3227 \begin_layout Itemize
3232 \begin_layout Examples
3237 \begin_layout Itemize
3238 \begin_inset Formula $\Lang{addition}\in\Class{AC}^{0}$
3244 \begin_layout Itemize
3245 \begin_inset Formula $\Lang{parity}\notin\Class{AC}^{0}$
3256 \begin_layout Column
3264 \begin_layout Plain Layout
3272 \begin_inset Formula $\Class{NC}^{1}$
3279 \begin_layout Plain Layout
3290 \begin_layout Itemize
3291 \begin_inset Formula $O(\log n)$
3297 \begin_layout Itemize
3302 \begin_layout Examples
3307 \begin_layout Itemize
3308 \begin_inset Formula $\Lang{parity}\in\Class{NC}^{1}$
3314 \begin_layout Itemize
3315 \begin_inset Formula $\Lang{mutiply}\in\Class{NC}^{1}$
3321 \begin_layout Itemize
3322 \begin_inset Formula $\Lang{divide}\in\Class{NC}^{1}$
3333 \begin_layout Column
3341 \begin_layout Plain Layout
3349 \begin_inset Formula $\Class{NC}^{2}$
3356 \begin_layout Plain Layout
3367 \begin_layout Itemize
3368 \begin_inset Formula $O(\log^{2}n)$
3374 \begin_layout Itemize
3379 \begin_layout Examples
3384 \begin_layout Itemize
3385 \begin_inset Formula $\Class{NL}\subseteq\Class{NC}^{2}$
3393 \begin_layout AgainFrame
3397 \begin_layout Plain Layout
3407 \begin_layout Subsection
3408 Standard Complexity Results on Finding Paths
3411 \begin_layout BeginFrame
3412 All Variants of Finding Paths in Directed Graphs
3413 \begin_inset Newline newline
3416 Are Equally Difficult
3420 \begin_inset Formula $\Lang{reach}$
3424 \begin_inset Formula $\Lang{distance}$
3428 \begin_inset Formula $\Class{NL}$
3439 \begin_layout Corollary
3440 For directed graphs, we can solve
3444 \begin_layout Itemize
3445 the reachability problem in logspace iff
3446 \begin_inset Formula $\Class{L}=\Class{NL}$
3452 \begin_layout Itemize
3453 the construction problem in logspace iff
3454 \begin_inset Formula $\Class{L}=\Class{NL}$
3460 \begin_layout Itemize
3461 the optimization problem in logspace iff
3462 \begin_inset Formula $\Class{L}=\Class{NL}$
3468 \begin_layout Itemize
3469 the approximation problem in logspace iff
3470 \begin_inset Formula $\Class{L}=\Class{NL}$
3477 \begin_layout AgainFrame
3481 \begin_layout Plain Layout
3491 \begin_layout BeginFrame
3492 FindingPaths in Forests and Directed Paths is Easy,
3493 \begin_inset Newline newline
3500 \begin_inset Formula $\Lang{reach}_{\operatorname{forest}}$
3504 \begin_inset Formula $\Lang{distance}_{\operatorname{forest}}$
3508 \begin_inset Formula $\Class{L}$
3514 \begin_layout Separator
3519 \begin_inset Formula $\Lang{reach}_{\operatorname{path}}$
3523 \begin_inset Formula $\Lang{distance}_{\operatorname{path}}$
3527 \begin_inset Formula $\Class{L}$
3533 \begin_layout AgainFrame
3537 \begin_layout Plain Layout
3547 \begin_layout Section
3548 Finding Paths in Tournaments
3551 \begin_layout Subsection
3552 Complexity of: Does a Path Exist?
3555 \begin_layout BeginFrame
3556 Definition of the Tournament Reachability Problem
3559 \begin_layout Definition
3565 \begin_inset Formula $\Lang{reach}_{\operatorname{tourn}}$
3573 \begin_inset Formula $(T,s,t)$
3580 \begin_layout Enumerate
3581 \begin_inset Formula $T=(V,E)$
3587 \begin_layout Enumerate
3588 there exists a path from
3589 \begin_inset space ~
3593 \begin_inset Formula $s$
3597 \begin_inset space ~
3601 \begin_inset Formula $t$
3608 \begin_layout BeginFrame
3609 The Tournament Reachability Problem is Very Easy
3612 \begin_layout Theorem
3613 \begin_inset Formula $\Lang{reach}_{\operatorname{tourn}}\in\Class{AC}^{0}$
3623 \begin_layout AlertBlock
3627 \begin_layout Plain Layout
3638 \begin_layout Itemize
3640 \begin_inset Quotes eld
3644 \begin_inset Quotes erd
3648 \begin_inset Formula $\Lang{reach}$
3652 \begin_inset Formula $\Lang{reach}_{\operatorname{path}}$
3658 \begin_layout Itemize
3659 \begin_inset Formula $\Lang{reach}\not\le_{\operatorname{m}}^{\Class{AC}^{0}}\Lang{reach}_{\operatorname{tourn}}$
3666 \begin_layout AgainFrame
3670 \begin_layout Plain Layout
3680 \begin_layout Subsection
3681 Complexity of: Construct a Shortest Path
3684 \begin_layout BeginFrame
3685 Finding a Shortest Path Is as Difficult as
3686 \begin_inset Newline newline
3689 the Distance Problem
3692 \begin_layout Definition
3698 \begin_inset Formula $\Lang{distance}_{\operatorname{tourn}}$
3706 \begin_inset Formula $(T,s,t,d)$
3713 \begin_layout Enumerate
3714 \begin_inset Formula $T=(V,E)$
3717 is a tournament in which
3720 \begin_layout Enumerate
3722 \begin_inset Formula $s$
3726 \begin_inset space ~
3730 \begin_inset Formula $t$
3734 \begin_inset space ~
3738 \begin_inset Formula $d$
3745 \begin_layout BeginFrame
3746 The Tournament Distance Problem is Hard
3749 \begin_layout Theorem
3750 \begin_inset Formula $\Lang{distance}_{\operatorname{tourn}}$
3754 \begin_inset Formula $\Class{NL}$
3760 \begin_layout Standard
3761 \begin_inset space \hfill{}
3768 \begin_layout Plain Layout
3772 hyperlink{hierarchy<6>}{
3774 beamerskipbutton{Skip Proof}}
3786 \begin_layout Corollary
3787 Shortest path in tournaments can be constructed
3788 \begin_inset Newline newline
3791 in logarithmic space, iff
3792 \begin_inset Formula $\Class{L}=\Class{NL}$
3802 \begin_layout Corollary
3803 \begin_inset Formula $\Lang{distance}\le_{\operatorname{m}}^{\Class{AC}^{0}}\Lang{distance}_{\operatorname{tourn}}$
3809 \begin_layout BeginFrame
3811 \begin_inset Formula $\Lang{distance}_{\operatorname{tourn}}$
3817 \begin_layout Standard
3821 \begin_layout Plain Layout
3833 \begin_layout Columns
3834 \begin_inset Argument
3837 \begin_layout Plain Layout
3847 \begin_layout Column
3851 \begin_layout Standard
3855 \begin_layout Plain Layout
3873 \begin_layout Plain Layout
3881 \begin_inset Formula $\Lang{reach}$
3885 \begin_inset Formula $\Lang{distance}_{\operatorname{tourn}}$
3892 \begin_layout Plain Layout
3903 \begin_layout Enumerate
3907 \begin_layout Plain Layout
3915 \begin_inset Formula $(G,s,t)$
3919 \begin_inset Formula $\Lang{reach}$
3925 \begin_layout Enumerate
3929 \begin_layout Plain Layout
3937 \begin_inset Formula $G$
3941 \begin_inset Formula $G'$
3947 \begin_layout Enumerate
3951 \begin_layout Plain Layout
3959 \begin_inset Newline newline
3963 \begin_inset Formula $(G',s',t',3)\in\Lang{distance}_{\operatorname{tourn}}$
3970 \begin_layout Separator
3978 \begin_layout Plain Layout
3989 \begin_layout Plain Layout
4000 \begin_layout Plain Layout
4011 \begin_layout Enumerate
4015 \begin_layout Plain Layout
4023 \begin_inset space ~
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4035 \begin_inset space ~
4039 \begin_inset Formula $G'$
4045 \begin_layout Enumerate
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4057 \begin_inset space ~
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4073 \begin_inset Formula $G'$
4080 \begin_layout Column
4084 \begin_layout Example
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5787 \begin_layout Plain Layout
5797 \begin_layout Subsection
5798 Complexity of: Approximating the Shortest Path
5801 \begin_layout BeginFrame
5802 Approximators Compute Paths that Are Nearly As Short As a Shortest Path
5805 \begin_layout Definition
5810 approximation scheme for
5811 \begin_inset Formula $\Lang{tournament-shortest-path}$
5822 \begin_layout Enumerate
5824 \begin_inset Formula $(T,s,t)\in\Lang{reach}_{\operatorname{tourn}}$
5830 \begin_layout Enumerate
5832 \begin_inset Formula $r>1$
5838 \begin_layout Standard
5842 \begin_layout Itemize
5844 \begin_inset Formula $s$
5848 \begin_inset space ~
5852 \begin_inset Formula $t$
5856 \begin_inset Formula $r\operatorname{d_{T}}(s,t)$
5863 \begin_layout BeginFrame
5864 There Exists a Logspace Approximation Scheme for
5865 \begin_inset Newline newline
5868 the Tournament Shortest Path Problem
5871 \begin_layout Theorem
5872 There exists an approximation scheme for
5873 \begin_inset Formula $\Lang{tournament-shortest-path}$
5877 \begin_inset Formula $1<r<2$
5881 \begin_inset Formula
5883 O\left(\log|V|\log\frac{1}{r-1}\right).
5895 \begin_layout Corollary
5896 In tournaments, paths can be constructed in logarithmic space.
5899 \begin_layout Standard
5900 \begin_inset space \hfill{}
5907 \begin_layout Plain Layout
5911 hyperlink{optimality}{
5913 beamergotobutton{More Details}}
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5925 \begin_layout Plain Layout
5935 \begin_layout EndFrame
5939 \begin_layout Standard
5940 \begin_inset Note Note
5943 \begin_layout Plain Layout
5944 If a frame includes a program listing, the frame needs to be marked as
5945 \begin_inset Quotes eld
5949 \begin_inset Quotes erd
5953 This is not yet supported by LyX, so the frame is created using TeX code.
5956 begin{frame}[fragile] needs to be preceeded by an
5968 \begin_layout Standard
5972 \begin_layout Plain Layout
5976 begin{frame}[fragile]
5984 \begin_layout Standard
5988 \begin_layout Plain Layout
5997 Just a frame with a program code listing
6001 \begin_layout Plain Layout
6011 \begin_layout Standard
6012 This is some program code:
6015 \begin_layout Standard
6016 \begin_inset listings
6017 lstparams "extendedchars=true,language=Python,numbers=left,stepnumber=3,tabsize=4"
6021 \begin_layout Plain Layout
6026 \begin_layout Plain Layout
6028 'this is a python function'
6031 \begin_layout Plain Layout
6036 \begin_layout Plain Layout
6041 \begin_layout Plain Layout
6043 'This is a German word: Tschüs'
6046 \begin_layout Plain Layout
6051 \begin_layout Plain Layout
6056 \begin_layout Plain Layout
6058 'this is a python function'
6061 \begin_layout Plain Layout
6071 \begin_layout Standard
6075 \begin_layout Plain Layout
6087 \begin_layout Section*
6091 \begin_layout Subsection*
6095 \begin_layout BeginFrame
6103 \begin_layout Plain Layout
6114 \begin_layout Itemize
6128 \begin_inset Formula $\Class{AC}^{0}$
6137 \begin_layout Itemize
6142 logspace approximation scheme
6154 shortest paths in tournaments.
6157 \begin_layout Itemize
6171 \begin_inset Formula $\Class{NL}$
6180 \begin_layout Separator
6188 \begin_layout Plain Layout
6199 \begin_layout Itemize
6200 The same results apply to graphs with
6201 \begin_inset Newline newline
6204 bounded independence number.
6205 \begin_inset space \hfill{}
6212 \begin_layout Plain Layout
6216 hyperlink{independence}{
6218 beamergotobutton{More Details}}
6226 \begin_layout Itemize
6227 The complexity of finding paths in undirected graphs
6228 \begin_inset Newline newline
6232 \begin_inset space \hfill{}
6239 \begin_layout Plain Layout
6243 hyperlink{undirected}{
6245 beamergotobutton{More Details}}
6254 \begin_layout Subsection*
6258 \begin_layout BeginFrame
6262 \begin_layout Standard
6266 \begin_layout Plain Layout
6270 beamertemplatebookbibitems
6278 \begin_layout Bibliography
6279 \labelwidthstring References
6280 \begin_inset CommandInset bibitem
6281 LatexCommand bibitem
6287 \begin_inset space ~
6295 \begin_layout Plain Layout
6306 Topics on Tournaments.
6313 \begin_layout Plain Layout
6322 Holt, Rinehart, and Winston, 1968.
6327 \begin_layout Plain Layout
6331 beamertemplatearticlebibitems
6339 \begin_layout Bibliography
6340 \labelwidthstring References
6341 \begin_inset CommandInset bibitem
6342 LatexCommand bibitem
6343 key "NickelsenT2002"
6348 \begin_inset space ~
6351 Arfst Nickelsen and Till Tantau.
6356 \begin_layout Plain Layout
6365 On reachability in graphs with bounded independence number.
6369 \begin_layout Plain Layout
6383 , Springer-Verlag, 2002.
6386 \begin_layout Bibliography
6387 \labelwidthstring References
6388 \begin_inset CommandInset bibitem
6389 LatexCommand bibitem
6395 \begin_inset space ~
6402 \begin_layout Plain Layout
6411 A logspace approximation scheme for the shortest path problem for graphs
6412 with bounded independence number.
6416 \begin_layout Plain Layout
6430 , Springer-Verlag, 2004.
6435 \begin_layout Plain Layout
6447 \begin_layout EndFrame
6451 \begin_layout Standard
6456 \begin_layout Plain Layout
6460 AtBeginSubsection[]{}
6468 \begin_layout Section
6472 \begin_layout Subsection
6473 Graphs With Bounded Independence Number
6476 \begin_layout BeginFrame
6480 \begin_layout Plain Layout
6482 [label=independence]
6487 Definition of Independence Number of a Graph
6490 \begin_layout Definition
6500 \begin_inset Formula $\alpha(G)$
6504 \begin_inset Newline newline
6507 is the maximum number of vertices we can pick,
6508 \begin_inset Newline newline
6511 such that there is no edge between them.
6514 \begin_layout Example
6515 Tournaments have independence number 1.
6519 \begin_layout BeginFrame
6520 The Results for Tournaments also Apply to
6521 \begin_inset Newline newline
6524 Graphs With Bounded Independence Number
6527 \begin_layout Theorem
6529 \begin_inset space ~
6533 \begin_inset Formula $k$
6544 in graphs with independence number
6545 \begin_inset Newline newline
6549 \begin_inset space ~
6553 \begin_inset Formula $k$
6557 \begin_inset Formula $\Class{AC}^{0}$
6563 \begin_layout Separator
6567 \begin_layout Theorem
6569 \begin_inset space ~
6573 \begin_inset Formula $k$
6580 logspace approximation scheme
6584 for approximating the shortest path in graphs with independence number at
6586 \begin_inset space ~
6590 \begin_inset Formula $k$
6596 \begin_layout Separator
6600 \begin_layout Theorem
6602 \begin_inset space ~
6606 \begin_inset Formula $k$
6617 in graphs with independence number at most
6618 \begin_inset space ~
6622 \begin_inset Formula $k$
6630 \begin_inset Formula $\Class{NL}$
6638 \begin_layout Subsection
6639 Finding Paths in Undirected Graphs
6642 \begin_layout BeginFrame
6646 \begin_layout Plain Layout
6648 <1-2>[label=undirected]
6653 The Complexity of Finding Paths in Undirected Graphs
6654 \begin_inset Newline newline
6661 \begin_inset Formula $\Lang{reach}_{\operatorname{undirected}}$
6665 \begin_inset Formula $\Class{SL}$
6671 \begin_layout Corollary
6672 For undirected graphs, we can solve
6676 \begin_layout Itemize
6677 the reachability problem in logspace iff
6678 \begin_inset Formula $\Class L=\Class{SL}$
6684 \begin_layout Itemize
6685 the construction problem in logspace iff
6689 \begin_layout Plain Layout
6707 \begin_layout Itemize
6708 the optimization problem in logspace iff
6712 \begin_layout Plain Layout
6730 \begin_layout Itemize
6731 the approximation problem in logspace iff ?.
6736 \begin_layout Subsection
6737 The Approximation Scheme is Optimal
6740 \begin_layout BeginFrame
6744 \begin_layout Plain Layout
6751 The Approximation Scheme is Optimal
6754 \begin_layout Theorem
6755 Suppose there exists an approximation scheme for
6756 \begin_inset Formula $\Lang{tournament-shortest-path}$
6760 \begin_inset Formula $O\bigl(\log|V|\log^{1-\epsilon}\frac{1}{r-1}\bigr)$
6765 \begin_inset Formula $\Class{NL}\subseteq\Class{DSPACE}\bigl[\log^{2-\epsilon}n\bigr]$
6776 \begin_layout Enumerate
6777 Suppose the approximation scheme exists.
6778 \begin_inset Newline newline
6782 \begin_inset Formula $\Lang{distance}_{\operatorname{tourn}}\in\Class{DSPACE}\bigl[\log^{2-\epsilon}n\bigr]$
6789 \begin_layout Enumerate
6791 \begin_inset Formula $(T,s,t)$
6796 \begin_inset Formula $n$
6799 be the number of vertices.
6802 \begin_layout Enumerate
6803 Run the approximation scheme for
6804 \begin_inset Formula $r:=1+\smash{\frac{1}{n+1}}$
6808 \begin_inset Newline newline
6812 \begin_inset Formula $\smash{O(\log^{2-\epsilon}n)}$
6818 \begin_layout Enumerate
6819 The resulting path has optimal length.
6824 \begin_layout Plain Layout
6837 \begin_layout EndFrame