TY - GEN
T1 - Settling the complexity of leontief and PLC exchange markets under exact and approximate equilibria
AU - Garg, Jugal
AU - Mehta, Ruta
AU - Vazirani, Vijay V.
AU - Yazdanbod, Sadra
N1 - Publisher Copyright:
© 2017 Copyright held by the owner/author(s).
PY - 2017/6/19
Y1 - 2017/6/19
N2 - Our first result shows membership in PPAD for the problem of computing approximate equilibria for an Arrow-Debreu exchange market for piecewise-linear concave (PLC) utility functions. As a corollary we also obtain membership in PPAD for Leontief utility functions. This settles an open question of Vazirani and Yannakakis (2011). Next we show FIXP-hardness of computing equilibria in Arrow-Debreu exchange markets under Leontief utility functions, and Arrow-Debreu markets under linear utility functions and Leontief production sets, thereby settling these open questions of Vazirani and Yannakakis (2011). As corollaries, we obtain FIXP-hardness for PLC utilities and for Arrow-Debreu markets under linear utility functions and polyhedral production sets. In all cases, as required under FIXP, the set of instances mapped onto will admit equilibria, i.e., will be "yes" instances. If all instances are under consideration, then in all cases we prove that the problem of deciding if a given instance admits an equilibrium is ETR-complete, where ETR is the class Existential Theory of Reals. As a consequence of the results stated above, and the fact that membership in FIXP has been established for PLC utilities, the entire computational difficulty of Arrow-Debreu markets under PLC utility functions lies in the Leontief utility subcase. This is perhaps the most unexpected aspect of our result, since Leontief utilities are meant for the case that goods are perfect complements, whereas PLC utilities are very general, capturing not only the cases when goods are complements and substitutes, but also arbitrary combinations of these and much more. Finally, we give a polynomial time algorithm for finding an equilibrium in Arrow-Debreu exchange markets under Leontief utility functions provided the number of agents is a constant. This settles part of an open problem of Devanur and Kannan (2008).
AB - Our first result shows membership in PPAD for the problem of computing approximate equilibria for an Arrow-Debreu exchange market for piecewise-linear concave (PLC) utility functions. As a corollary we also obtain membership in PPAD for Leontief utility functions. This settles an open question of Vazirani and Yannakakis (2011). Next we show FIXP-hardness of computing equilibria in Arrow-Debreu exchange markets under Leontief utility functions, and Arrow-Debreu markets under linear utility functions and Leontief production sets, thereby settling these open questions of Vazirani and Yannakakis (2011). As corollaries, we obtain FIXP-hardness for PLC utilities and for Arrow-Debreu markets under linear utility functions and polyhedral production sets. In all cases, as required under FIXP, the set of instances mapped onto will admit equilibria, i.e., will be "yes" instances. If all instances are under consideration, then in all cases we prove that the problem of deciding if a given instance admits an equilibrium is ETR-complete, where ETR is the class Existential Theory of Reals. As a consequence of the results stated above, and the fact that membership in FIXP has been established for PLC utilities, the entire computational difficulty of Arrow-Debreu markets under PLC utility functions lies in the Leontief utility subcase. This is perhaps the most unexpected aspect of our result, since Leontief utilities are meant for the case that goods are perfect complements, whereas PLC utilities are very general, capturing not only the cases when goods are complements and substitutes, but also arbitrary combinations of these and much more. Finally, we give a polynomial time algorithm for finding an equilibrium in Arrow-Debreu exchange markets under Leontief utility functions provided the number of agents is a constant. This settles part of an open problem of Devanur and Kannan (2008).
KW - ETR
KW - FIXP
KW - Leontief
KW - Market equilibria
KW - PLC
KW - PPAD
UR - http://www.scopus.com/inward/record.url?scp=85024407615&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85024407615&partnerID=8YFLogxK
U2 - 10.1145/3055399.3055474
DO - 10.1145/3055399.3055474
M3 - Conference contribution
AN - SCOPUS:85024407615
T3 - Proceedings of the Annual ACM Symposium on Theory of Computing
SP - 890
EP - 901
BT - STOC 2017 - Proceedings of the 49th Annual ACM SIGACT Symposium on Theory of Computing
A2 - McKenzie, Pierre
A2 - King, Valerie
A2 - Hatami, Hamed
PB - Association for Computing Machinery
T2 - 49th Annual ACM SIGACT Symposium on Theory of Computing, STOC 2017
Y2 - 19 June 2017 through 23 June 2017
ER -