Page 2 of 2

Re: Theory Tea : 2011 - 2012

Posted: Fri Nov 04, 2011 1:33 pm
by Loner
alexpsomi wrote:Αφού δεν με διαφημίζετε εσείς θα διαφημιστώ μόνος μου :-p
Spoiler: εμφάνιση/απόκρυψη
[quote]Ayth thn Paraskeuh tha mas milhsei o Alexandros Psomas gia mia koinh mas douleia, pou sthrizetai sthn ptuxiakh tou ergasia, kai h opoia prosfata egine dekth gia dhmosieysh se ena apo ta sunedria ths perioxhs mas. Epistrefoume sthn aithousa A36, omws auth thn Paraskeuh tha einai stis 5 anti gia tis 4 pou einai sinithws. Title: On Worst-Case Allocations in the Presence of Indivisible Goods Abstract: We study a fair division problem, where a set of indivisible goods is to be allocated to a set of n agents. Each agent may have different preferences, represented by a valuation function that is a probability distribution on the set of goods. In the continuous case, where goods are infinitely divisible, it is well known that proportional allocations always exist, i.e., allocations where every agent receives a bundle of goods worth to him at least 1/n. In the presence of indivisible goods however, this is not the case and one would like to find worst case guarantees on the value that every agent can have. We focus on algorithmic and mechanism design aspects of this problem. In the work of Hill 1987, an explicit lower bound was identified, as a function of the number of agents and the maximum value of any agent for a single good, such that for any instance, there exists an allocation that provides at least this guarantee to every agent. The proof however did not imply an efficient algorithm for finding such allocations. Following upon the work of Hill, we first provide a slight strengthening of the guarantee we can make for every agent, as well as a polynomial time algorithm for computing such allocations. We then move to the design of truthful mechanisms. For deterministic mechanisms, we obtain a negative result showing that a truthful 2/3-approximation of these guarantees is impossible. We complement this by exhibiting a simple truthful algorithm that can achieve a constant approximation when the number of goods is bounded. Regarding randomized mechanisms, we also provide a negative result, showing that we cannot have truthful in expectation mechanisms under the restrictions that they are Pareto-efficient and satisfy certain symmetry requirements.
[/quote]
I'll be there!! :)

Re: Theory Tea : 2011 - 2012

Posted: Fri Nov 04, 2011 1:38 pm
by cypher
Loner wrote: I'll be there!! :)
Έγινε ήδη την προηγούμενη παρασκευή αυτό. :lol: :lol:

Re: Theory Tea : 2011 - 2012

Posted: Fri Nov 04, 2011 2:50 pm
by Loner
Image

Re: Theory Tea : 2011 - 2012

Posted: Sun Nov 06, 2011 6:54 pm
by alexpsomi
Loner wrote: I'll be there!! :)
Image

Re: Theory Tea : 2011 - 2012

Posted: Thu Nov 10, 2011 12:02 pm
by enum21
Auth thn Paraskeuh tha mas milhsei o Giwrgos Zois gia th douleia pou
kanei sto didaktoriko tou,

O titlos einai: "Speed Scaling for Maximum Lateness"

A36 stis 4

Re: Theory Tea : 2011 - 2012

Posted: Wed Nov 30, 2011 2:31 pm
by enum21
Thn Paraskeuh stis 4, sthn A36 ws sinithws,
Tha mas milhsei h Giwta Panagopoulou apo to CTI (Computer Technology
Institute) ths Patras

Τίτλος: Χρωματισμοί Γραφημάτων και Θεωρία Παιγνίων

Περίληψη:
Ένα ενδιαφέρον πεδίο της αλγοριθμικής θεωρίας παιγνίων είναι να μελετηθεί κατά πόσο μπορεί η θεωρία παιγνίων να βοηθήσει στην ανάπτυξη και ανάλυση αλγορίθμων για υπολογιστικά δύσκολα προβλήματα συνδυαστικής βελτιστοποίησης. Προς την κατεύθυνση αυτή, θα μελετήσουμε από παιγνιοθεωρητική σκοπιά το πρόβλημα χρωματισμού των κορυφών ενός γραφήματος. Θα ορίσουμε κατάλληλα ένα στρατηγικό παίγνιο (που ονομάζουμε παίγνιο χρωματισμού γραφήματος) με σύνολο παικτών το σύνολο των κορυφών του γραφήματος και σύνολο στρατηγικών για κάθε κορυφή ένα κοινό σύνολο χρωμάτων, όπου το κέρδος μιας κορυφής είναι 0 αν υπάρχει γειτονική κορυφή με το ίδιο χρώμα, διαφορετικά ισούται με το πλήθος των κορυφών που έχουν επιλέξει το ίδιο χρώμα με τη συγκεκριμένη κορυφή. Η ανάλυση των ιδιοτήτων των ισορροπιών Nash του παιγνίου χρωματισμού γραφήματος οδηγεί σε μια σειρά από άνω φράγματα στο συνολικό πλήθος των χρωμάτων που χρησιμοποιούνται. Όλα αυτά τα άνω φράγματα αποτελούν ταυτόχρονα άνω φράγματα στο χρωματικό αριθμό του γραφήματος, και ο αλγόριθμος εύρεσης
μιας αγνής ισορροπίας Nash που θα παρουσιάσουμε εγγυάται ότι μπορούμε να υπολογίσουμε σε πολυωνυμικό χρόνο έναν ορθό χρωματισμό του γραφήματος που χρησιμοποιεί συνολικά ένα πλήθος χρωμάτων που ικανοποιεί ταυτόχρονα
τα περισσότερα κλασικά γνωστά φράγματα στο χρωματικό αριθμό.

Re: Theory Tea : 2011 - 2012

Posted: Sun Dec 04, 2011 5:00 pm
by XaviannNJ
Ανεβαίνουν πουθενα οι διαφάνειες των παρουσιάσεων;

Re: Theory Tea : 2011 - 2012

Posted: Sun Dec 04, 2011 8:47 pm
by enum21
XaviannNJ wrote:Ανεβαίνουν πουθενα οι διαφάνειες των παρουσιάσεων;
Όσες έχουν ανέβει (και από προηγούμενα έτη) είναι εδώ.

Re: Theory Tea : 2011 - 2012

Posted: Sun Dec 04, 2011 9:18 pm
by XaviannNJ
enum21 wrote:
XaviannNJ wrote:Ανεβαίνουν πουθενα οι διαφάνειες των παρουσιάσεων;
Όσες έχουν ανέβει (και από προηγούμενα έτη) είναι εδώ.
Ευχαριστώ! :)

Re: Theory Tea : 2011 - 2012

Posted: Thu Dec 15, 2011 8:12 pm
by mpatsis
Για να αναστήσω το Thread!!!
Την προηγούμενη Παρασκεύη είχαμε μια πολύ ενδιαφέρουσα παρουσίαση από τον Jonathan Παναγιωτίδη!!! :-D

Αυτή την Παρασκεύη δεν θα έχει Theory Tea. Θα έχει όμως την Δευτέρα 19/12/2011 στις 17:00 και ο ομιλητής θα είναι ο γνωστός σε όλους μας Χρήστος Παπαδημητρίου. Να είστε όλοι εκεί. ( Αίθουσα Α36 )
19/12/2011: Christos Papadimitriou, Market Equilibria Monday A36 17:00

Re: Theory Tea : 2011 - 2012

Posted: Thu Dec 15, 2011 8:14 pm
by chriskin
καθολου ασχημα :smt023

Re: Theory Tea : 2011 - 2012

Posted: Thu Dec 15, 2011 8:22 pm
by mpatsis
chriskin wrote:καθολου ασχημα :smt023
Join us my friend! ;)
They serve cookies, coffee, tea and many more for you to eat.
And above all knowledge!!! Knowledge is power!!! :-D :-D :-D

Re: Theory Tea : 2011 - 2012

Posted: Thu Dec 15, 2011 8:29 pm
by chriskin
tea > knoledge = power > cookies > coffee

ελπιζω να μην βαρεθω κι αυτη τη φορα :oops:
δεν πεφτει πανω στο αναλογο του foss ομως; μηπως να αλλαζαμε ωρα σε εκεινο μπας και το βρω σαν ευκαιρια και τα παρακολουθησω ολα; :lol:

Re: Theory Tea : 2011 - 2012

Posted: Thu Jan 19, 2012 11:22 pm
by mpatsis
δεν πεφτει πανω στο αναλογο του foss ομως; μηπως να αλλαζαμε ωρα σε εκεινο μπας και το βρω σαν ευκαιρια και τα παρακολουθησω ολα; :lol:
I know. Και εγώ θέλω να πάω και στα 2! Αλλά foss έχει κάθε εβδομάδα...
Θα το πω στα παιδιά αν μπορούν να το αλλάξουν (δύσκολο πάντως γιατί πρέπει και να μπορούν και να βρουν και αίθουσα κλπ).
---------------------
Αύριο έχει tea???

Re: Theory Tea : 2011 - 2012

Posted: Thu Jan 19, 2012 11:25 pm
by Gewitter
όχι!είχε την Τετάρτη!

Re: Theory Tea : 2011 - 2012

Posted: Thu Jan 19, 2012 11:34 pm
by mpatsis
Gewitter wrote:όχι!είχε την Τετάρτη!
Ένα ποστ να ρίχνετε πλιζ!!!

Re: Theory Tea : 2011 - 2012

Posted: Tue Mar 13, 2012 7:42 pm
by cypher
Geia se olous,

Auth thn Paraskeuh epistrefoume
me omilhtria thn Katia Papakonstantinopoulou, apo to Panepisthmio Athinwn.

Stis 4, sthn A36.
Akolouthei titlos kai abstract

Title: Contention issues in congestion games

Abstract:
Communication networks, due to the competitive nature of their users, are widely studied using Game Theory. A key tool in analyzing such networks, modeled as games, is the study of their stable states, that is, their equilibria. Understanding the properties of equilibria can provide insights into the effectiveness of economic policies, engineering decisions, etc.

Among the communication networks of interest, there is an important class in which the players can time their participation in the game with the hope that fewer players will compete for the same resources. Motivated by these games, we study time-dependent strategies for playing congestion games. In this talk I will discuss some recent advances.

I will first present the two models we introduce: (i) the boat model, in which the latency of a player is influenced only by the players that start at the same time, and (ii) the conveyor belt model, in which the latency of a player is affected by the players that share the system, even if they started earlier or later.

Then the structural properties of the games of both models will be described. I will show that the games of the boat model are themselves congestion games. The same is true for the games of two players for the conveyor belt model; however, for this model the games of three or more players are not in general congestion games and may not have pure equilibria.
Consequently, the boat games do possess pure Nash equilibria and the exact topology (the order of the edges in the paths) of the network does not affect the latency of the players, while the conveyor belt games the network topology does matter and they may not have pure Nash equilibria for three or more players.

Finally I will characterize the symmetric Nash equilibria and optimal solutions of boat games of networks of parallel links with affine latencies and conveyor belt games of two players on parallel links with arbitrary latency functions, and give bounds on their price of anarchy and stability.

This is joint work with Elias Koutsoupias.
----------------------------------------------------------------
Exei prokupsei mia allagh sthn wra twn parousiasewn,

H omilia tha einai thn Paraskeuh stis 5 sthn aithousa A36 kai oxi stis 4,

Katia Papakonstantinopoulou, University of Athens,
Title: Contention issues in congestion games

Abstract:
Communication networks, due to the competitive nature of their users, are widely studied using Game Theory. A key tool in analyzing such networks, modeled as games, is the study of their stable states, that is, their equilibria. Understanding the properties of equilibria can provide insights into the effectiveness of economic policies, engineering decisions, etc.

Among the communication networks of interest, there is an important class in which the players can time their participation in the game with the hope that fewer players will compete for the same resources. Motivated by these games, we study time-dependent strategies for playing congestion games. In this talk I will discuss some recent advances.

I will first present the two models we introduce: (i) the boat model, in which the latency of a player is influenced only by the players that start at the same time, and (ii) the conveyor belt model, in which the latency of a player is affected by the players that share the system, even if they started earlier or later.

Then the structural properties of the games of both models will be described. I will show that the games of the boat model are themselves congestion games. The same is true for the games of two players for the conveyor belt model; however, for this model the games of three or more players are not in general congestion games and may not have pure equilibria.
Consequently, the boat games do possess pure Nash equilibria and the exact topology (the order of the edges in the paths) of the network does not affect the latency of the players, while the conveyor belt games the network topology does matter and they may not have pure Nash equilibria for three or more players.

Finally I will characterize the symmetric Nash equilibria and optimal solutions of boat games of networks of parallel links with affine latencies and conveyor belt games of two players on parallel links with arbitrary latency functions, and give bounds on their price of anarchy and stability.

This is joint work with Elias Koutsoupias.

Re: Theory Tea : 2011 - 2012

Posted: Tue Apr 03, 2012 10:11 am
by cypher
Auth thn Paraskeuh tha mas milhsei o Giannhs Mpelimezakhs sxetika me to thema ths diplwmatikhs tou ergasias.
H omilia einai stis 5, alla sthn aithousa A41, kai oxi sthn A36 pou eimaste sinithws.

Title: On max-min fair allocations of indivisible goods

Abstract
In this problem we have m players and n goods and we want to allocate the goods to the players in a fair way. Every player i has a utility function u_i which expresses his valuation for every good. We focus on additive valuation functions, which means that the total utility of a player for a set of goods is the sum of the utility derived for each good in the set. Our goal is to find an allocation of the goods so that the utility of the least happy player is maximized. This is typically referred to as a max-min fair allocation.

The problem is NP-hard even for 2 identical agents. Hence a series of approximation algorithms have been proposed in the literature for various special cases. In this talk, we will focus on the special case of identical agents, i.e., all agents have the same utility function. We will present a Polynomial Time Approximation Scheme (PTAS) for this case due to G. Woeginger.
--------------------
Auth thn ebdomada to theory tea tha ginei Pempth anti gia Paraskeuh,
stis 17:00 kai sthn aithousa A24.

Tha mas milhsei o k. Fotakis apo to EMP gia themata mhxanismwn se facility location problems. Akolouthei titlos kai abstract.

------------------------------------------------------------------
Speaker: Dimitris Fotakis, Lecturer, National Technical University of Athens

TITLE: Deterministic strategyproof mechanisms for Facility Location Games

ABSTRACT: We investigate the approximability of K-Facility Location by deterministic strategyproof mechanisms. Our main result is an elegant characterization of deterministic strategyproof mechanisms with a bounded approximation ratio for 2-Facility Location on the line. In particular, we show that for instances with at least 5 agents, any such mechanism either admits a unique dictator, or always places the facilities at the two extremes.

Thus, we obtain that the best approximation ratio achievable by deterministic strategyproof mechanisms for 2-Facility Location on the line is precisely n-2, where n is the number of agents. Another rather surprising consequence is that the {\sc Two-Extremes} mechanism of \cite{PT09} is the only deterministic anonymous strategyproof mechanism with a bounded approximation ratio for 2-Facility Location on the line.

The proof of the characterization employs several new ideas and technical tools, which provide new insights into the behavior of deterministic mechanisms for multiple Facility Location games, and may be of independent interest.

Employing one of these tools, we show that for every K \geq 3, there do not exist any deterministic anonymous strategyproof mechanisms with a bounded approximation ratio for K-Facility Location, even for simple instances with K+1 agents.

This is joint work with Christos Tzamos.

Re: Theory Tea : 2011 - 2012

Posted: Thu Apr 05, 2012 1:12 am
by cypher
theorytea-group wrote:Gia osous tha einai Athina thn ebdomada amesws meta to Pasxa,
tha thelame na sas ypenthumisoume oti diorganwnoume fetos sto OPA
to ISCO 2012 (2nd International Symposium on Combinatorial Optimization), 19-21/4.
Akribws prin to sunedrio (17-18/4) tha yparxei ena polu endiaferon spring school se approximation algorithms apo tous kateksoxhn experts sto pedio auto, David Shmoys kai David Williamson apo to Cornell University.

To programma kai oles oi leptomereies einai sth selida:
http://isco12.cs.aueb.gr/

Eiste oloi euprosdektoi! (den xreiazetai na kanete to registration gia na parakolouthisete tis omilies)

Re: Theory Tea : 2011 - 2012

Posted: Wed Apr 25, 2012 6:33 pm
by cypher
markarkis wrote:Auth thn Paraskeuh tha mas milhsei o Christos Alatzidis gia to thema ths diplwmatikhs tou sta plaisia tou metaptyxiakou programmatos Episthmhs Ypologistwn.

Sthn A36 stis 17:00. Akolouthei titlos kai abstract.

Title: An Introduction to Combinatorial Auctions

Abstract:
We will present an introduction to combinatorial auctions and truthful mechanism design. We will start by defining simple single-item auctions, multi-item auctions and combinatorial auctions (multiple items, multiple bidders, combinatorial valuations) and we will then continue by defining and stating the significance of truthful mechanism design. We will present the Vickrey auction and the payment functions that are admitted. Furthermore, we will focus on bidders with submodular utility functions and show a greedy 2-approximation algorithm for this setting. We will also define a more restricted class of submodular valuations usually referred to as additive valuations with budget constraints and examine the behavior of the greedy algorithm under this domain.

Re: Theory Tea : 2011 - 2012

Posted: Thu May 10, 2012 10:07 am
by cypher
Αυτή την Παρασκευή θα μας μιλήσει η Κωνσταντίνα Γκαλμπογκίνη σχετικά
με τη διπλωματική της,
η οποία αφορά τη μελέτη κοινωνικών δικτύων,

Στις 17:00, στην αίθουσα Α36.

Abstract:
Τα κοινωνικά δίκτυα όπως το Twitter και το Google+, είναι μια εικόνα
της κοινωνίας μας και αποτελούν ένα πολύ ενεργό ερευνητικό πεδίο από
την ακαδημαϊκή κοινότητα τα τελευταία χρόνια. Παρουσιάζουμε ανασκόπηση
των γραφοθεωρητικών χαρακτηριστικών του δικτύου Twitter που έχουν
μελετηθεί στην βιβλιογραφία, καθώς και μετρικές επιρροής κόμβων στο
σύνολο του δικτύου και αλγορίθμους επιλογής των κόμβων με την
μεγαλύτερη επιρροή ανά θέμα. Εφαρμόζουμε τις μετρικές αυτές σε δικές
μας συλλογές από ελληνικά δεδομένα των δικτύων Twitter και Google+ και
προτείνουμε νέο αλγόριθμο επιλογής των κόμβων με επιρροή στο δίκτυο.
Συζητάμε, τέλος, τις δυσκολίες συλλογής δεδομένων από τα δίκτυα αυτά

Re: Theory Tea : 2011 - 2012

Posted: Wed May 30, 2012 1:30 pm
by cypher
Ayth thn Parskeuh stis 5 tha mas parousiasei o Giannhs Mpelimezakhs th
diplwmatikh tou,
akolouthoun leptomereies parakatw

---------------------------------------------------------------------------------
Την Παρασκευή 1/06/2012 στην αίθουσα Α36 και ώρα 17:00, θα γίνει η
παρουσίαση της διπλωματικής του κ. Ι. Μπελιμεζάκη.
Τίτλος: Δίκαιη κατανομή αγαθών

Επιβλέπων κ. Ι. Μήλης

2ος Αξιολογητής κ. Ε. Μαρκάκης

Περίληψη: Στο πρόβλημα της δίκαιης κατανομής αδιαίρετων αγαθών
δίνονται m παίκτες, n αγαθά και μία συνάρτηση αποτίμησης u_j(i)
κάθε αγαθού i από κάθε παίκτη j. Δεδομένης μίας ανάθεσης A των αγαθών
στους παίκτες, η συνολική αποτίμηση κάθε παίκτη, U_j(Α), είναι το
άθροισμα των αποτιμήσεων των αγαθών που του ανατέθηκαν. Στόχος είναι η
ανάθεση των αγαθών στους παίκτες με δίκαιο τρόπο, δηλαδή η
μεγιστοποίηση της συνολικής αποτίμησης του λιγότερου ευχαριστημένου
παίκτη ή διαφορετικά max_A min_j U_j(A).Το πρόβλημα της δίκαιης
κατανομής αγαθών είναι ΝP-hard ακόμη και για δύο πανομοιότυπους
παίκτες.
Στη διπλωματική παρουσιάζονται θετικά και αρνητικά αποτελέσματα
προσεγγισιμότητας του προβλήματος που έχουν εμφανιστεί στη
βιβλιογραφία τα τελευταία χρόνια. Τα αποτελέσματα αφορούν τόσο την
γενική περίπτωση του προβλήματος που είναι γνωστή και ως πρόβλημα του
Άγιου Βασίλη (u_j(i)>=0), όσο και ειδικότερες περιπτώσεις που
περιλαμβάνουν πανομοιότυπους παίκτες (u_j(i)=u(i)), ομοιόμορφα
σχετιζόμενους παίκτες (u_j(i)=u(i)/s_j) και άλλες ειδικές συναρτήσεις
αποτίμησης όπως u_j(i) \in {0,u(i)}, u_j(i) \in {0,1,x}, u_j(i) \in
{0,1, ∞} και u_j(i) \in {0,1}. Επίσης παρουσιάζεται ένα πολυωνυμικού
χρόνου προσεγγιστικό σχήμα (PTAS) για την ειδική περίπτωση των 2
παικτών.