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We analyze a game of strategic experimentation with two-armed bandits whose risky arm might yield payoffs after exponentially distributed random times. Free-riding causes an inefficiently low level of experimentation in any equilibrium where the players use stationary Markovian strategies with beliefs as the state variable. We construct the unique symmetric Markovian equilibrium of the game, followed by various asymmetric ones. There is no equilibrium where all players use simple cut-off strategies. Equilibria where players switch finitely often between experimenting and free-riding all yield a similar pattern of information acquisition, greater efficiency being achieved when the players share the burden of experimentation more equitably. When players switch roles infinitely often, they can acquire an approximately efficient amount of information, but still at an inefficient rate. In terms of aggregate payoffs, all these asymmetric equilibria dominate the symmetric one wherever the latter prescribes simultaneous use of both arms. Copyright The Econometric Society 2005.

Inelastic neutron scattering from a single crystal of the quasi-two-dimensional antiferromagnet has been used to measure the spin wave dispersion curve at 4 K. The exchange integrals were subsequently calculated from linear spin wave theory. The values meV, meV, meV and meV are within stability conditions calculated from mean-field theory. In addition, the critical behaviour of the gap in the spin wave energy at the Brillouin zone centre has been measured, and compared to the critical behaviour of the magnetization from neutron scattering data of the magnetic (020) Bragg peak. The gap varies with magnetization for , and with the square of the magnetization for . Two possible explanations are proposed: a competition between single-ion and dipolar anisotropies; or a crossover to XY-like excitations.

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This paper studies optimal experimentation by a monopolist who faces an unknown demand curve subject to random changes, and who maximizes profits over an infinite horizon in continuous time. We show that there are two qualitatively very different regimes, determined by the discount rate and the intensities of demand curve switching, and the dependence of the optimal policy on these parameters is discontinuous. One regime is characterized by extreme experimentation and good tracking of the prevailing demand curve, the other by moderate experimentation and poor tracking. Moreover, in the latter regime the agent eventually becomes ''trapped'' into taking actions in a strict subset of the feasible set.