REVIEW 4 major objections 3 minor 30 references
Hawkes-Driven OTC Market Making: Volterra-Riccati Approximation
T0 review · 4 major / 3 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read The paper shows that a state-feedback Volterra–Riccati quote rule, driven by the conditional forecast of future RFQ flow, closely approximates the exact optimal market-making policy for Hawkes-distributed requests and generates an endogenou
desk verdict A solid approximation paper for Hawkes-driven OTC market making, with a strong exponential benchmark and an honest but unvalidated long-memory section—worth refereeing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the Volterra forecast curve m_t(u) = E[λ_{t+u} | F_t], a deterministic curve that summarizes the Hawkes memory. A hierarchy of approximations is built on it: the mean Volterra–Riccati surrogate solves a backward Riccati system with m_t as the driving intensity; the noise-aware version adds a covariance correction; the state-feedback version replaces m_t with the post-request forecast m_t + ρ_i, using the functional sensitivity of the value map to form the post-request shadow price. Quotes are always produced by the exact Hamiltonian optimizer applied to the approximated shadow price. In the exponential benchmark the forecast curve is represented by the finite-dimensional Haw
What would settle it
Solve the power-law-like Hawkes model with a high-fidelity numerical method (e.g., a very large exponential mixture lift, or a path-dependent PDE solver) and compare the resulting optimal quotes and value against the state-feedback Volterra–Riccati policy; if the relative regret grows with the kernel tail exponent, the approximation's long-memory claims fail. Alternatively, from real RFQ data, measure the decay of quote skew after a directional burst and check whether it matches the fitted kernel's tail exponent.
Extended reading notes
Core claim
The central discovery is that the path-dependent Hawkes market-making problem, which needs the full RFQ history as state, can be reduced to a tractable quote rule operating on the conditional forecast curve of future request intensities. The paper proves numerically, in an exponential Hawkes benchmark where the exact lifted HJB can be solved, that the state-feedback Volterra–Riccati policy — which updates the forecast curve after each request and recomputes the quadratic continuation value — closely tracks the exact optimal quotes, with relative regret around 0.08%–0.17% across the tested regimes, while a Poisson policy that ignores memory loses up to 27%. The same rule, when applied to a po
Load-bearing premise
The receding-horizon conditional surrogate is assumed to remain an accurate approximation of the true path-dependent value function for general long-memory kernels, but exact validation is only supplied for exponential kernels, and Section 7 explicitly states that a general error or convergence theory remains open.
Editorial extensions
If this is right
- Ignoring request-flow history when flow is directional leads to systematically misplaced quotes and larger inventory risk, even if the dealer solves the misspecified Poisson problem optimally.
- The state-feedback Volterra–Riccati rule provides a closed, low-dimensional quoting algorithm that remains applicable when exact Markovian lifting is impractical, such as power-law or very high-dimensional mixture kernels.
- An observed directional RFQ burst should alter quotes persistently, not just momentarily, because the conditional forecast of future flow is itself persistent; the quote skew then decays with the forecast response tail.
- Conditioning on the RFQ memory primarily improves risk control (reduces inventory and P&L variance), more than it changes mean P&L, in the long-memory experiment.
Reading between the lines
- One could use the linearized impact formula (50) to invert observed quote-skew decay into an estimate of the Hawkes memory kernel, giving a new empirical test on RFQ data.
- The approximation hierarchy suggests a modular production design: a Hawkes/Volterra forecast engine, a Riccati solver, and a tabulated Hamiltonian optimizer can replace a full dynamic-programming solver at low latency.
- If the forecast response decays as a power law, the model predicts OTC quote impact should also decay as a power law with the same exponent; this is testable with dealer-level or platform-level RFQ data.
- The method could be extended to treat win probabilities as state-dependent (e.g., competition or toxicity), but then the state-feedback update would need to propagate through the response function as well; the paper's validation does not cover that case.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops a Volterra–Riccati approximation hierarchy for OTC market making when RFQ arrivals are modelled by general Hawkes kernels. It sets up an exact path-dependent HJB, then replaces the history state with conditional future-intensity forecast curves and uses a quadratic/Riccati expansion, adding a covariance correction and a state-feedback update after observed RFQs. In an exponential Hawkes benchmark, the state-feedback policy is shown numerically to closely track the exact lifted HJB, while a memory-free Poisson policy suffers large regret. The same rule is then applied to a power-law-like N=8 exponential mixture; a directional RFQ burst produces a persistent quote skew and reduces inventory/P&L risk relative to a Poisson benchmark. The paper explicitly states that no general error or convergence theory is provided and that the frozen conditional forecast does not satisfy an intertemporal dynamic-programming principle.
Significance. The exponential validation is a genuine strength: the exact Markovian lift is solved and compared with common random numbers and paired regrets, and the state-feedback mechanism is shown to matter, especially in directional regimes. If the approximation can be trusted for long-memory kernels, the paper offers a tractable bridge between persistent RFQ flow and market-making controls. However, the headline long-memory application is not yet supported to the same standard. The quote-impact tail in Eq. (53) is inherited by construction from the forecast response in the approximate rule, and Section 6.2 lacks any non-Poisson reference solution; these gaps must be addressed before the long-memory claims can be accepted.
major comments (4)
- [Section 6.2, Section 7] The power-law experiment cannot support the central long-memory claim because the only comparator is the Poisson policy. Section 6.2 states that no lifted HJB is solved for the N=8 exponential mixture, and Section 7 concedes that 'a general error or convergence theory remains open' and that the frozen conditional forecast does not satisfy an intertemporal dynamic-programming principle. Consequently Figs. 5–7 establish properties of the approximate state-feedback rule, not of the exact optimal policy; an approximate policy can beat a misspecified Poisson policy while being far from the true path-dependent optimum. Please add a reference benchmark or error calibration in this regime, e.g., an exact lifted HJB for a smaller exponential mixture (N=2 or N=3) under the same economic parameters, a convergence study in N_f and N, or an a posteriori bound using the covariance/curvature terms in S
- [Section 6.1, Eqs. (50)–(53)] The long-memory impact tail is hardwired into the approximate rule. Eq. (50) defines the incremental shadow-price impact as a finite-lag convolution of the resolvent response rho_{i0} with the sensitivity difference D(t,q;u)-D(t,q+epsilon_i z_i;u). If that sensitivity kernel is integrable, the convolution automatically preserves the tail order of rho, so Eq. (53) is a mathematical consequence of the definition of the state-feedback rule, not a testable prediction of the exact optimal policy. The abstract's wording, 'endogenous OTC quote impact inherits the long-memory decay of the RFQ forecast response', is therefore accurate for the approximate policy, but it should be labelled as such; an independent optimality argument would be needed to claim that the true optimal policy has this property.
- [Section 4.3, Eqs. (36)–(38), (42), (50)] The noise-aware correction and the linearized impact formulas require twice functional differentiability of the map m -> V0(t,q;m), which is assumed but not proved or even stated with conditions. Since V0 is obtained from a Riccati system with coefficients linear in m, the derivatives are likely available under mild integrability conditions, but the paper does not derive the resulting sensitivity ODEs or state the required function space for m. This matters for Eq. (50), where integrability in u of the sensitivity difference is precisely what allows the tail to be pulled through the convolution. Please state the assumptions and verify the derivative equations used in the implementation, at least in the exponential benchmark where the exact lifted HJB is available.
- [Section 5.1, numerical truncation] The validation relies on truncated inventory and memory grids (qmax=50, Xmax=800, 1000 memory points) but no grid-convergence or truncation-sensitivity study is reported. Since the exact benchmark is itself discretized, the claims about 0.08–0.17% relative regret for the state-feedback policy should be accompanied by a check that the discretization is fine enough and that the truncation does not bias the comparison. This is not a fatal issue, but it is needed to make the numerical validation fully convincing.
minor comments (3)
- [Section 2, Table 1] The fitted branching ratios and mixture weights are reported without standard errors, confidence intervals, or goodness-of-fit diagnostics, and the data are proprietary. Since Table 1 is the empirical motivation for the entire model, even a brief indication of estimation uncertainty would help the reader assess the strength of the persistence evidence.
- [Section 5.1 and Section 6.2] There are small notational inconsistencies in the sigmoid win-probability formulas: the parameterization in Section 5.1 (f(δ) = (1+exp(δ-1.2))^{-1}) and in Section 6.2 (f(δ) = (1+exp(3(δ-1)))^{-1}) differ, and the displayed formula in Section 5.1 has a parenthesis imbalance. These should be cleaned up.
- [Section 7] The paper is commendably explicit about its limitations, but the concluding section could more directly state that the power-law results in Section 6 are properties of the approximate policy, not of the exact solution of the path-dependent HJB, unless the proposed convergence/benchmark work is added.
Circularity Check
No significant circularity: the power-law impact tail is an explicitly derived model implication, and the exponential benchmark provides an independent validation reference; the only self-citation is non-load-bearing.
full rationale
The derivation chain is self-contained. The exact path-dependent HJB, Eq. (18), is a genuine optimal-control formulation, and the Volterra–Riccati objects in Eqs. (23)–(43) are explicitly labeled as conditional surrogates: the paper states, e.g., in Section 4.2, that Eq. (23) 'is not claimed to be the HJB equation of the original Hawkes control problem, nor a time-consistent auxiliary value function.' The exponential benchmark in Section 5 is a meaningful external check: the state-feedback rule is compared against a numerically solved lifted HJB, which is not constructed from the approximation, and the reported closeness is a nontrivial numerical result. The power-law impact section is an analytic consequence rather than a fitted prediction. Section 6.1 defines the incremental shadow-price impact, Eq. (50), as a Volterra filter of the resolvent response rho_i, and then derives the tail behavior in Eqs. (52)–(53). The text explicitly says 'the impact is therefore a Volterra filter of the RFQ forecast response' and that 'the time decay is inherited from the RFQ-memory kernel.' This is an openly derived model implication, not an empirical result that has been fitted to data and then re-reported as a discovery. The Poisson-benchmark comparison in Section 6.2 is a controlled numerical illustration of the value of conditioning, not a hidden circular step. The only self-citation (Barzykin 2026) is a passing literature contrast — 'complementary to the inventory-based impact of a trade' — and is not load-bearing. Section 7's concession that 'a general error or convergence theory remains open' is an honest limitation on the approximation, not a circularity. No circular step meeting the evidence threshold was found.
Assumptions & free parameters
free parameters (5)
- Fitted Hawkes branching ratios and mixture weights =
eta 0.862-0.895; weights w1, w10, w60 (Table 1)
- Power-law kernel mixture (branching matrix and exponent) =
B = [[0.55,0.05],[0.05,0.55]], nu_mix=0.65, N=8, log-spaced decay rates
- Win probability sigmoid parameters =
Benchmark: slope 1, offset 1.2; power-law: slope 3, offset 1
- Risk aversion and terminal penalty =
Benchmark kappa=0.2, kappa_T=0.2; power-law kappa=2.0, kappa_T=0.01
- Horizon and discretization parameters =
T=1 day (benchmark), Teval=0.1, Tcont=0.1, grid sizes as in Section 5.1
assumptions (6)
- domain assumption The decomposition of the dealer objective into spread revenue minus quadratic inventory risk (14) is the correct reduced control objective under martingale mid-price.
- domain assumption Request arrivals are exogenous and independent of the dealer's quotes; fills do not feed back into future RFQ intensity.
- ad hoc to paper The quadratic Hamiltonian expansion (25) and quadratic inventory ansatz (26) accurately approximate the continuation value.
- ad hoc to paper The map from forecast curve m to mean Volterra-Riccati value V0 is twice functionally differentiable (36).
- ad hoc to paper For power-law kernels, the Riccati sensitivity kernel in (50) is integrable and finite-horizon effects are negligible so the impact tail matches the Hawkes response tail (52)-(53).
- ad hoc to paper Existence and uniqueness of the path-dependent HJB (18) is assumed.
Cite this review
Pith. "Pith review of Hawkes-Driven OTC Market Making: Volterra-Riccati Approximation." pith.science (2026). https://pith.science/paper/34X7222N
@misc{pith2026260802002,
author = {Pith},
title = {Pith review of: Hawkes-Driven OTC Market Making: Volterra-Riccati Approximation},
year = {2026},
howpublished = {\url{https://pith.science/paper/34X7222N}},
note = {Machine review of arXiv:2608.02002}
}
read the original abstract
We formulate an over-the-counter (OTC) market-making problem in which request-for-quote (RFQ) arrivals are modelled by general Hawkes kernels and fills are controlled thinnings of the exogenous request flow. The modelling choice is motivated by spot-FX RFQ data: after filtering and transforming to seasonality-adjusted RFQ activity time, two-way activity in major currency pairs exhibits large fitted branching ratios and multi-scale persistence. For general Hawkes kernels the control problem is path-dependent: the relevant state contains the order-flow history, or equivalently the forward curve of conditional future RFQ intensities. Exact Markovian lifting is available for exponential kernels, but it becomes high-dimensional for mixtures of exponentials and impractical for long-memory kernels. We therefore develop a hierarchy of Volterra-Riccati approximations. The first level replaces random future request flow by its conditional Volterra forecast; the second adds a covariance correction for intensity uncertainty; the third updates the quote rule with the realized Hawkes memory, or equivalently with the post-request conditional forecast curve. The approximation hierarchy is validated in an exponential Hawkes benchmark, where the exact lifted HJB can be solved numerically. The state-feedback Volterra-Riccati policy closely tracks the exact benchmark, especially in directional regimes, while a memory-free Poisson policy suffers substantial regret. We then apply the same state-feedback rule to a power-law-like RFQ memory model. A directional RFQ burst changes the conditional forecast of future flow and is converted by the continuation-value shadow price into a persistent quote skew. The resulting endogenous OTC quote impact inherits the long-memory decay of the RFQ forecast response and improves inventory and P&L risk control relative to a no-conditioning Poisson benchmark.
Figures
Figures from the paper (4 more)
Reference graph
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Reviewed August 4, 2026 · model on record in the stance chip above.
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