This supplement contains additional results and proofs omitted from the main paper.
S.1. ZERO AGGREGATE DEMAND IN EQUILIBRIUMIN THIS SECTION, we formally derive equilibria in the special examples in which aggregate demand ends up being zero on the equilibrium path (footnote 12 in Section 2.2 and footnote 20 in Section 5). In Section S.1.1, we work out the example with one informed trader, corresponding to footnote 12, and in Section S.1.2, we work out the example with multiple informed traders, corresponding to footnote 20. S.1.1. One Informed Trader EXAMPLE S.1: The value of the security, v, is distributed normally with mean 0 and variance 1. There is one strategic trader with signal θ 1 who observes the value perfectly: θ 1 = v. The demand of liquidity traders is u = −v. The market maker does not observe any signals beyond the aggregate demand.This example satisfies Assumptions 1 and 2 in Section 3, and thus we can use the closedform solutions derived in the proof of Theorem 1 and presented in Section 3.2 of the paper (for the special case k M = 0). Because we have only one strategic trader in the example, many of the matrices become scalars, simplifying the calculation.Specifically, Σ θθ = Σ diag = 1, and, therefore, Λ = 2 and Λ −1 = 1/2. Next, Σ θv = 1, while This example also satisfies Assumptions 1 and 2 in Section 2, and thus we can use the closed-form solutions derived in the proof of Theorem 1 and presented in Section 3.2 (again, for the special case k M = 0). We now have multiple strategic traders, so the calculations involve matrix manipulations.Specifically, Σ θθ is an m-dimensional matrix whose elements are all equal to 1, while Σ diag is an m-dimensional identity matrix. We thus haveThe coefficients of the quadratic equation on γ are, therefore2 , and c = 1/(m + 1) 2 , which in turn gives us γ = 1/m and β D = m. Thus,and so on the equilibrium path, aggregate demand is equal toThus, for every m, the market price on the equilibrium path is also always equal to 0, not revealing any information contained in the signals of the strategic traders and in the demand coming from the liquidity traders.
S.2. ZERO INTERCEPTS IN EQUILIBRIUMIn this section, we prove the statement made informally in footnote 13 in Section 3.2 that in our setting, linear equilibria with nonzero intercepts do not exist.
PROPOSITION S.1: Suppose there exists an equilibrium of the formThen β 0 = 0 and for all i, δ i = 0.PROOF: Consider a particular realization of θ i , θ M , and u. Then in this equilibrium, the realized price will be given byBy the definition of equilibrium, for every realization of θ M and D, the price set by the market maker is equal to the expected value of the security conditional on θ M and D:Integrating over all possible realizations of θ M and D, we thus get, for the unconditional expectation of the price,Since by assumption, E[v] = 0, and also E[θ M ], E[u], and E[θ i ] (for all i) are equal to 0, by taking the unconditional expectation of Equation (S.1), we getNow, as inStep 2 of the proof of Theorem 1, consi...