Why loan trading can change bank capital
Banks that can sell loans after origination may face different incentives to screen them before lending. A buyer that has invested in screening can inspect a seller's loan and buy it when its quality is high. That opportunity can make screening more valuable and reduce the equity a buyer needs to commit. At the same time, a seller may prefer to avoid the screening cost and rely on a later sale. Song and Thakor study when these two choices coexist in an interbank market, even though banks begin with the same opportunities and information (Song and Thakor, SSRN 3667175). The resulting question concerns the capital structure of banks and whether a trading market opens a region of heterogeneous behavior between two homogeneous regimes.
The paper's Proposition 2 is a result about this transition as the chance of a boom changes. It also compares the width of the heterogeneous region across market efficiencies. The mission formalizes that result from the authors' specified model and their preprint's assumptions. The preprint describes the paper as forthcoming in Management Science; the DOI record identifies the associated journal work.
The model and its quantities
A good loan pays X>I at its final date. A medium loan pays X in a boom and zero in a recession. Both require an initial investment I. The probability of a boom is θ∈[0,θˉ], with θˉ<I/X, so a medium loan is not creditworthy on its own. Before a bank screens, each type has probability one half. Screening costs c and reveals the type. A bank can finance the loan with deposits and equity; supplying one unit of equity costs insiders ψ per unit. Default also loses the bank its charter value Y(θ). The paper restricts X,I,c,δ,ψ,θˉ, and Y by Assumptions 1–4 and fixes the matching technology by Assumption 5 (preprint, pp. 7–12); Y is increasing and concave.
Without loan trading, the minimum screening equity is Ehom(θ), the profit from screening is πshom(θ), and the profit without screening is πnshom(θ). Their difference, Δπhom(θ), is the screening advantage. The paper writes each quantity as an explicit real-valued function of θ in equations (8)–(11). A cutoff θ∗ separates the choices when the loan market is closed (Proposition 1, p. 15).
With trading, some banks can be buyers that screen, while others are sellers that do not. If buyers have mass Nb and sellers mass Ns, market tightness is n=Nb/Ns. A seller meets a buyer with probability α(n)=λn/(1+n); a buyer meets a seller with probability λ/(1+n). Here λ∈[0,1] measures matching efficiency. The buyer's bargaining weight is 1/(1+n), and the price for a verified good loan is p(n)=X−(X−I)/(1+n). Trading reduces the buyer's required equity to Ehet(θ,n)=Ehom(θ)−λ(X−p(n))/(1+n) (Assumption 5, equation (13), and Lemma 1; the matching function follows Kiyotaki and Wright (1993)).
Formalization targets
The main target is Proposition 2, p. 21. Under the paper's assumptions and its endpoint conditions, there are cutoffs 0<θL<θH<θˉ. Every bank screens below θL; none screens above θH; between them, buyers and sellers coexist. In the interior the buyer/seller ratio is determined by
λ(X−I)[21−θn+1n−1−(n+1)2ψ]=Δπhom(θ).
The two cutoffs solve the paper's equations (A4) and (A7). The mission also targets uniqueness of the equilibrium in every regime, expansion of the heterogeneous region when a larger feasible λ is used, and falling n and p(n) as θ rises within that region. The milestone list covers the matching identities of Assumption 5, the Nash price (13), Lemma 1's equity threshold, the monotonicity of Ehom (footnote 19), the sign form of the cost condition (A1), the strict concavity of Δπhom, the closed-market cutoff of Proposition 1, the two trading cutoffs (A4) and (A7), the interior indifference equation (A9)/(15), and the monotonicity in n of the left side of (15) (Appendix A, pp. 34–37).
What the result establishes
The result identifies a region in which banks that started alike choose different capital structures and screening decisions. A loan-trading market changes the relative payoffs of screening and not screening, and the boom probability determines which choice can survive a unilateral switch. The cutoffs show precisely where the market moves from all-screening to mixed behavior and then to no-screening. The market-efficiency comparison says when the mixed region widens. These distinctions matter for the paper's subsequent welfare comparison and its discussion of capital requirements (Song and Thakor, §§3–4).
The result is proved in the source paper. The remaining work is a machine-checked proof of the model-specific statements and their consequences, including the bargaining maximum, incentive threshold, cutoff existence and uniqueness, and comparative statics. The definitions are reusable for further formal work on the paper's welfare and capital-requirement results; this mission does not assert that those extensions have been formalized.
Main mathematical difficulty
The equilibrium claim joins several regimes in one statement. The homogeneous corners are governed by whether a single bank would gain by changing its behavior, whereas the interior is governed by equality of buyer and seller profits. A root of the displayed equation alone does not establish an equilibrium: the cutoff and no-switch conditions must also hold, and the root must be unique. Moreover, the source's claimed global negative derivative for Δπhom is not implied by its assumptions. The cutoff argument must therefore use a valid shape condition without importing that false assertion. The charter-value function Y is concave but otherwise general, so reasoning cannot rely on a particular polynomial example.
Formalization scope
Lean represents all expectations by the closed-form real expressions in equations (7)–(15); there is no probability space. The parameter structure fixes X,I,c,δ,ψ,λ,θˉ, and Y. All five numbered assumptions remain explicit, including Assumption 4. The formalization reads screening cost and equity cost as positive, reads the paper's smooth Y as differentiable on [0,θˉ], and requires λ>0 for an open market. The bargaining price is optimized over [I,X]. The buyer meeting probability uses its continuous value λ at n=0. Proposition 1 uses a reduced closed-market outcome that records screening and equity; at its profit tie, footnote 21 selects no screening. The open-market equilibrium variable is the buyer share s∈[0,1], with n=s/(1−s) only in the interior; the all-buyer corner uses the sole-seller limit instead. Price and buyer equity are constructed from the paper's equations, and are never arbitrary choices. The cutoffs are required to satisfy 0<θL<θH<θˉ, so all three regimes are nonempty; a version in which the equilibrium is any root of (15), or in which price or buyer equity are free variables, would be a different and weaker statement and is excluded. The hypotheses are jointly satisfiable, but only for small market efficiency (numerical searches find λ≲0.015); an explicit rational instance is checked in Lean.
The preprint prints a plus sign in (A6) where its preceding deviation inequality requires a minus sign; the Lean condition uses the latter. The paper also claims that the absolute sensitivity of n to θ falls with λ, but that claim is false under its hypotheses, so the mission does not include either sensitivity clause of Proposition 2. The paper's unsupported global first-derivative bound for Δπhom is excluded. The milestone from the proof of Proposition 5 states its displayed second-derivative identity wherever Y′′ exists, and states strict concavity throughout the interval. Any further result should keep these corrections explicit. Contributions to the bargaining, real-analysis, and equilibrium lemmas are welcome; proofs of the milestone statements can then support the capstone theorem.
Selected references
- N. Kiyotaki and R. Wright, A search-theoretic approach to monetary economics, American Economic Review 83(1), 63–77, 1993. JSTOR
- F. Song and A. V. Thakor, Market Freeze and Bank Capital Structure Heterogeneity, SSRN preprint 3667175, version dated 2022-02-21; forthcoming in Management Science in the source artifact. SSRN; DOI.