REVIEW 4 major objections 5 minor 2 cited by
Temperature dependence in Krylov space
T0 review · 4 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read Two Toda chains drive Lanczos coefficients as temperature changes
desk verdict Two Toda chains for beta-dependence of Lanczos coefficients is real and worth engaging, but 'full generality' in the abstract oversells: the closed two-chain flow requires the no-degenerate-gaps condition, and there is a factor-2 slip in the K~e^{-beta m/2} claim. 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 central objects are the tridiagonal Liouvillian matrix M (representing [H,·] in the Krylov basis) and the matrix T representing the superoperator -1/2{H,·}; their commutativity [M,T]=0, together with the QR decomposition of e^{T(β-β0)/2}, produces the Lax equations (2.23). The even and odd parts of T decouple into two Toda chains, related only through their conserved quantities at the initial condition. The eigenvalues of T are the energy sums -1/2(E_i+E_j), and the late-time diagonalization theorem for Toda flows is what yields the asymptotic Lanczos coefficients and the exponential decay of Krylov complexity.
What would settle it
Take a small system with no degenerate energy gaps and an off-diagonal initial operator (e.g. the four-state model of Sec 2.1.2), compute b_n(β) by direct Lanczos at β and β+δ, and compare the numerical derivative to the right-hand sides of (2.53)–(2.54); any discrepancy beyond integration error would falsify the closed Toda flow. Alternatively, run the same check on the harmonic oscillator: the paper itself shows the closed equations fail there because the extra term Y in (2.118) is nonzero, so the oscillator cleanly delimits the claim's domain.
Extended reading notes
Core claim
The central claim is that the β-dependence of Lanczos coefficients b_n(β) is governed by the Lax pair dT/dβ=[B,T], dM/dβ=[B,M] with B=(T_+−T_-)/2, where M is the Liouvillian in the Krylov basis and T represents -1/2{H,·}. After projecting on even and odd indices, this splits into two independent Toda chains whose only relation is the equality of conserved quantities at the initial condition. In the large-β limit βm≫1, the Toda flow forces T to diagonal form, so b_{2k+1}→0 and b_{2k}→|E_i−E_j|; the time-averaged Krylov complexity then decays as e^{-βm/2}. The paper also introduces the "Krylov bootstrap": consistency of the flow with the known CFT Lanczos coefficients b_n^2=(n+1)(n+2Δ)π^2/β^2
Load-bearing premise
The derivation assumes that moving an operator toward lower temperature, by conjugating with e^{-βH/2}, never leaves the original Krylov space—true only when the operator has no matrix elements between equal-energy states and no on-diagonal part; if that fails, the equations for the Lanczos coefficients are not closed, and the paper's CFT conclusion relies on an asserted mismatch that is not shown.
Editorial extensions
If this is right
- Thermal correlators at any β can be obtained by integrating the Lax equations from β=0 Lanczos data, bypassing a fresh Lanczos run at each temperature.
- At very low temperature, the Krylov chain effectively truncates: odd coefficients vanish, even coefficients encode the energy gaps, so Lanczos data becomes a spectral probe of |E_i−E_j|.
- Time-averaged Krylov complexity is exponentially small in βm/2 in the low-temperature limit, essentially independent of system size.
- The Krylov bootstrap turns the flow equations into a consistency test on candidate spectral functions: any proposed b_n(β) that cannot satisfy the closed Lax equations within the Krylov space signals either degeneracies or a nonzero diagonal part of the operator.
Reading between the lines
- One could use the Krylov bootstrap in reverse: given measured or conjectured Lanczos coefficients at two temperatures, failure of the closed Lax equations would pinpoint the presence of degenerate energy gaps or of a diagonal component in the operator.
- The late-time Toda diagonalization suggests a way to extract the energy-gap spectrum from the even Lanczos coefficients at low temperature; testing this on larger interacting systems would show whether the β→∞ limit is reached before finite-size effects dominate.
- The β=0 numerical integration scheme, if it scales, would offer an alternative route to finite-temperature dynamics that does not require imaginary-time evolution or matrix-product-state thermalisation; the paper only demonstrates five sites, so scalability is the open question.
- The paper's distinction between two staggering mechanisms (spectral gap m vs constant part κ) predicts that systems with the same spectral function shape but different κ will show different branch splittings; this is directly testable in exactly solvable models like the XY chain.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the temperature (β) dependence of Lanczos coefficients b_n(β) for thermal two-point functions defined with the Wightman inner product. The authors derive Lax equations for the tridiagonal matrix M representing the Liouvillian and for the matrix T representing the superoperator -1/2{H,·} in Krylov space, under the condition that this superoperator does not move operators outside the Krylov space. In that no-degenerate-gaps setting, they show that even and odd Lanczos coefficients decouple into two independent Toda chains related via initial conditions. They apply this structure to the large-β limit, where half the Lanczos coefficients vanish and the other half approach energy gaps, and to the moderate-β regime, where they explain staggering via spectral gaps and delta-function weight in the spectral function. They also introduce a 'Krylov bootstrap' consistency argument, claiming that the known CFT Lanczos coefficients force a degenerate spectrum, and propose a numerical method to compute C_β(t) from β=0 data.
Significance. If the main derivation stands, this is a valuable contribution: it connects the recursion-method/Lanczos-coefficient formalism to the Toda hierarchy, gives an integrable interpretation of β-dependence, and produces concrete analytic predictions for low temperatures, including the exponential suppression of Krylov complexity. The paper contains clean derivations in Sec. 2.1, exact solvable examples (Examples I–III), and a numerical demonstration of the flow method in Sec. 3.2. These are genuine strengths. However, the advertised 'full generality' is not achieved: the two-Toda-chain result holds only under a restrictive no-degenerate-gaps condition, and several of the application-level claims rest on asserted or heuristic steps. These issues are fixable but require substantive revision.
major comments (4)
- [Abstract; Sec. 2.1, 2.3] The abstract states the result 'in full generality,' but the derivation of the two independent Toda chains requires T⊥_n = 0 in Eq. (2.11), i.e. A has no matrix elements connecting degenerate energy levels. This excludes every operator with nonzero diagonal part and any system with degenerate nonzero gaps. When this condition fails, the flow of the physical Lanczos coefficients is not closed: Eqs. (2.116)–(2.118) involve t or Y, which are not determined by b_n alone. The harmonic oscillator example in Appendix B makes this explicit: T_K satisfies (B.2) only with an external Y. The paper should either restrict the central claim to the no-degenerate-gaps case or present the extended-space Lax dynamics as the general integrable statement.
- [Sec. 3.1, after Eq. (3.4)] The Krylov bootstrap conclusion—that the explicit CFT Lanczos coefficients b_n^2=(n+1)(n+2Δ)π^2/β^2 force a degenerate spectrum—is asserted, not demonstrated. The sentence 'Evaluating the right hand side of (2.76), one finds it not to match the left hand side' is a load-bearing step. Please display the explicit mismatch (at least for the first non-trivial n), or provide a supplementary computation, so the claim is verifiable.
- [Sec. 3.4, Eq. (3.20)] There is an internal inconsistency in the exponential decay. The text states b_0^2 ≈ e^{-βm/2}; substituting into Eq. (3.19) with b_1 finite gives ln K ≈ -βm/2 + O(1), not -βm. As written, Eq. (3.20) contradicts the abstract, the introduction, and Fig. 2. Please correct Eq. (3.20) to ln K ≈ -βm/2, or if the intended asymptotic is -βm, change the stated b_0^2 scaling accordingly.
- [Sec. 3.3, Eqs. (3.7)–(3.9)] The large-β diagonalization argument is used to derive b_{2k+1}→0 and b_{2k}→|E_i-E_j|, which underlies all low-temperature predictions. The argument is heuristic: the boundedness of the partial sums in Eq. (3.7) is assumed rather than shown, the eigenvalue-ordering argument around Eq. (3.8) is a perturbation statement, and the claim that the doubly degenerate eigenvalues are ordered with doublets consecutive requires justification. Since these conclusions are central, please either supply a precise statement with the relevant convergence theorem from Refs. [15,16] or explicitly mark this part as heuristic.
minor comments (5)
- [Sec. 2.1.1, Eqs. (2.33)–(2.35)] The notation T versus \tilde{T} is confusing; the reader must track which matrix is tridiagonalized and which evolves in the original basis. Please define the relationship once and consistently.
- [Eq. (2.25)] The termination condition for finite Krylov space is stated parenthetically. Please spell out the index ranges and the convention for b_n=0 for n≥N, since this formula is used repeatedly.
- [Eq. (3.2)] Typo: there is a stray comma in 'b_n^2 = (n+1)(n+2Δ)π^2/β^2, .' Remove the comma.
- [Fig. 2 caption] The fitting constant is denoted '#'; use a conventional symbol such as c or A to avoid confusion.
- [Appendix C] Typo: 'Lnczos' should be 'Lanczos'.
Circularity Check
No significant circularity: the Lax/Toda equations are derived from first principles; assumptions and self-citations are non-load-bearing.
full rationale
The paper's central derivation is self-contained. Equations (2.13)-(2.23) construct the temperature-evolution operator from the finite-temperature inner product, use the Jacobi-identity commutator [M,T]=0, and obtain the Lax pair via QR decomposition; nothing is fitted or assumed to be a Toda chain. The even/odd split into two Toda chains (2.33)-(2.35) follows from the parity structure T_{nm}=0 for n+m odd, not from an ansatz. The large-β asymptotics in Sec. 3.3 import external Toda convergence theorems (Moser; Deift-Li-Tomei) and are heuristic around the fixed point, but they do not reduce to the paper's own inputs. The K ~ e^{-βm/2} claim in Sec. 3.4 uses the truncated-chain formula (3.19) and the gapped spectral model (4.4) for b_0^2 ~ e^{-βm/2}; only the prefactor is fitted in Fig. 2, not the exponential exponent. The 'Krylov bootstrap' in Sec. 3.1 relies on the CFT Lanczos coefficients (3.2) from [14]; although [14] shares an author, the formula is parameter-free and externally checkable from the CFT two-point function (3.1), so it is independent support rather than load-bearing self-citation. The paper explicitly flags the no-degenerate-gaps restriction after Eq. (2.9) and the extended-Krylov data Y,t needed in Sec. 2.3; these are scope limitations on 'full generality', not circular reductions. The asserted mismatch in Sec. 3.1 is not displayed, but an omitted computation is a proof gap, not circularity.
Assumptions & free parameters
free parameters (5)
- omega_max (UV cutoff in model spectral function (4.4)) =
not specified; varied (e.g., Fig 3a)
- kappa (zero-frequency weight) =
computed from thermal 1pt function; e.g., kappa = m_z^2 for XY model (Sec C)
- Delta (UV singularity exponent) =
conformal dimension or UV scaling exponent of Phi(omega)
- fitting constant # for e^{-beta m/2} =
fit to data in Fig 2
- Euler step Delta_beta =
3.3e-4
assumptions (6)
- domain assumption Wightman inner product choice rho1 = rho2 = e^{-beta H/2} (eq 2.3)
- domain assumption No degenerate nonzero energy gaps and vanishing diagonal part of A (T_perp = 0)
- domain assumption Generic energy-sum spectrum: each -(E_i+E_j)/2 is exactly doubly degenerate and doublets appear consecutively in decreasing order.
- standard math Toda flow converges to diagonal form at late times (Moser; Deift-Li-Tomei)
- ad hoc to paper Hard-cutoff spectral model (4.4): Phi(omega) = (1/N)e^{-beta|omega|/2} + kappa delta(omega) on m < |omega| < omega_max
- standard math CFT Lanczos coefficients b_n^2 = (n+1)(n+2Delta)pi^2/beta^2 from ref [14] and exactness of the recursion solution (3.4)
Cite this review
Pith. "Pith review of Temperature dependence in Krylov space." pith.science (2026). https://pith.science/paper/P6LYPSDT
@misc{pith2026250819233,
author = {Pith},
title = {Pith review of: Temperature dependence in Krylov space},
year = {2026},
howpublished = {\url{https://pith.science/paper/P6LYPSDT}},
note = {Machine review of arXiv:2508.19233}
}
read the original abstract
We consider the recursion method applied to a generic 2pt function of a quantum system and show, in full generality, that the temperature dependence of the corresponding Lanczos coefficients is governed by integrable dynamics. After an appropriate change of variables, Lanczos coefficients with even and odd indices are described by two independent Toda chains, related at the level of the initial conditions. Consistency of the resulting equations can be used to show that certain scale-invariant models necessarily have a degenerate spectrum. We dub this self-consistency-based approach the ''Krylov bootstrap''. The known analytic behavior of the Toda chain at late times translates into analytic control over the 2pt function and Krylov complexity at very low temperatures. We also discuss the behavior of Lanczos coefficients when the temperature is low but not much smaller than the spectral gap, and elucidate the origin of the staggering behavior of Lanczos coefficients in this regime.
Forward citations
Cited by 2 Pith papers
-
Krylov Complexity from Loschmidt Amplitude
Krylov complexity is the ϕ-derivative of a Loschmidt amplitude and is upper-bounded by the volume of the induced (t,ϕ) Fubini-Study geometry.
-
QCommute: a tool for symbolic computation of nested commutators in quantum many-body spin-1/2 systems
QCommute is a new C++ tool for algebraic symbolic computation of nested commutators in quantum spin-1/2 many-body systems on hypercubic lattices in the thermodynamic limit.
Reference graph
Works this paper leans on
-
[1]
P. Nandy, A.S. Matsoukas-Roubeas, P. Martinez-Azcona, A. Dymarsky and A. del Campo, Quantum dynamics in krylov space: Methods and applications , Physics Reports 1125-1128 (2025) 1
work page 2025
-
[2]
E. Rabinovici, A. S´ anchez-Garrido, R. Shir and J. Sonner, Krylov Complexity , 2507.06286
- [3]
-
[4]
A. Avdoshkin, A. Dymarsky and M. Smolkin, Krylov complexity in quantum field theory, and beyond, JHEP 06 (2024) 066 [ 2212.14429]. – 31 –
arXiv 2024
-
[5]
A. Dymarsky and A. Gorsky, Quantum chaos as delocalization in krylov space , Phys. Rev. B 102 (2020) 085137
work page 2020
-
[6]
D.J. Yates, A.G. Abanov and A. Mitra, Dynamics of almost strong edge modes in spin chains away from integrability , Phys. Rev. B 102 (2020) 195419
work page 2020
-
[7]
D.J. Yates, A.G. Abanov and A. Mitra, Lifetime of almost strong edge-mode operators in one-dimensional, interacting, symmetry protected topological phases , Phys. Rev. Lett. 124 (2020) 206803
work page 2020
-
[8]
D.J. Yates and A. Mitra, Strong and almost strong modes of floquet spin chains in krylov subspaces, Phys. Rev. B 104 (2021) 195121
work page 2021
Show all 41 references
-
[9]
Yeh and A
H.-C. Yeh and A. Mitra, Universal model of Floquet operator Krylov space , Phys. Rev. B 110 (2024) 155109 [ 2311.15116]
2024 arXiv
-
[10]
Camargo, V
H.A. Camargo, V. Jahnke, K.-Y. Kim and M. Nishida, Krylov complexity in free and interacting scalar field theories with bounded power spectrum , Journal of High Energy Physics 2023 (2023)
2023
-
[11]
Deift, L
P. Deift, L. Li, T. Nanda and C. Tomei, The toda flow on a generic orbit is integrable , Communications on pure and applied mathematics 39 (1986) 183
1986
-
[12]
Bloch and S.N
A.M. Bloch and S.N. Karp, Symmetric toda, gradient flows, and tridiagonalization , Physica D: Nonlinear Phenomena 450 (2023) 133766
2023
-
[13]
Kac and P
M. Kac and P. van Moerbeke, On an explicitly soluble system of nonlinear differential equations related to certain toda lattices , Advances in Mathematics 16 (1975)
1975
-
[14]
Dymarsky and M
A. Dymarsky and M. Smolkin, Krylov complexity in conformal field theory , Phys. Rev. D 104 (2021) L081702
2021
-
[15]
Moser, Finitely many mass points on the line under the influence of an exponential potential – an integrable system , in Dynamical Systems, Theory and Applications , J
J. Moser, Finitely many mass points on the line under the influence of an exponential potential – an integrable system , in Dynamical Systems, Theory and Applications , J. Moser, ed., vol. 38, pp. 467–497 (1975), DOI
1975
-
[16]
L.L. P. Deift and C. Tomei, Toda flows with infinitely many variables , Journal of Functional Analysis 64 (1985)
1985
-
[17]
Chihara, An Introduction to Orthogonal Polynomials , Dover (2011)
T.S. Chihara, An Introduction to Orthogonal Polynomials , Dover (2011)
2011
-
[18]
V. Uvarov, The connection between systems of polynomials orthogonal with respect to different distribution functions , USSR Computational Mathematics and Mathematical Physics 9 (1969) 25
1969
-
[19]
Gamayun, M.A
O. Gamayun, M.A. Mir, O. Lychkovskiy and Z. Ristivojevic, Exactly solvable models for universal operator growth, 2504.03435
-
[20]
Bartsch, A
C. Bartsch, A. Dymarsky, M.H. Lamann, J. Wang, R. Steinigeweg and J. Gemmer, Estimation of equilibration time scales from nested fraction approximations , Physical Review E 110 (2024)
2024
-
[21]
Yates, A.G
D.J. Yates, A.G. Abanov and A. Mitra, Long-lived period-doubled edge modes of interacting and disorder-free Floquet spin chains , Commun. Phys. 5 (2022) 43 [ 2105.13766]
2022 arXiv
-
[22]
Tang, Operator Krylov complexity in random matrix theory , 2312.17416
H. Tang, Operator Krylov complexity in random matrix theory , 2312.17416
-
[23]
C. Tan, Z. Wei and R. Zhang, Scaling relations of spectral form factor and krylov complexity at finite temperature, Phys. Rev. E 111 (2025) 014135. – 32 –
2025
-
[24]
Chakraborty, Topics in Quantum Information, Complexity and Chaos Author , Ph.D
D. Chakraborty, Topics in Quantum Information, Complexity and Chaos Author , Ph.D. thesis, University of Kentucky, 2024
2024
-
[25]
Balasubramanian, J.M
V. Balasubramanian, J.M. Magan and Q. Wu, Quantum chaos, integrability, and late times in the krylov basis , Phys. Rev. E 111 (2025) 014218
2025
-
[26]
Vasli, K.B
M.J. Vasli, K.B. Velni, M.R.M. Mozaffar, A. Mollabashi and M. Alishahiha, Krylov complexity in lifshitz-type scalar field theories , The European Physical Journal C 84 (2024)
2024
-
[27]
Anegawa, N
T. Anegawa, N. Iizuka and M. Nishida, Krylov complexity as an order parameter for deconfinement phase transitions at large N , JHEP 04 (2024) 119 [ 2401.04383]
2024 arXiv
-
[28]
Chattopadhyay, V
A. Chattopadhyay, V. Malvimat and A. Mitra, Krylov complexity of deformed conformal field theories, JHEP 08 (2024) 053 [ 2405.03630]
2024 arXiv
-
[29]
He and H.-Q
P.-Z. He and H.-Q. Zhang, Probing Krylov Complexity in Scalar Field Theory with General Temperatures, 2407.02756
-
[30]
Aguilar-Gutierrez, H.A
S.E. Aguilar-Gutierrez, H.A. Camargo, V. Jahnke, K.-Y. Kim and M. Nishida, Krylov operator complexity in holographic CFTs: Smeared boundary reconstruction and the dual proper radial momentum, 2506.03273
-
[31]
Dodelson, Ringdown in the SYK model , 2408.05790
M. Dodelson, Ringdown in the SYK model , 2408.05790
-
[32]
Dodelson, Black holes from chaos , 2501.06170
M. Dodelson, Black holes from chaos , 2501.06170
-
[33]
Vidal, Efficient simulation of one-dimensional quantum many-body systems , Phys
G. Vidal, Efficient simulation of one-dimensional quantum many-body systems , Phys. Rev. Lett. 93 (2004) 040502
2004
-
[34]
Verstraete, J.J
F. Verstraete, J.J. Garcia-Ripoll and J.I. Cirac, Matrix product density operators: Simulation of finite-temperature and dissipative systems , Physical review letters 93 (2004) 207204
2004
-
[35]
Sugiura and A
S. Sugiura and A. Shimizu, Thermal pure quantum states at finite temperature , Phys. Rev. Lett. 108 (2012) 240401
2012
-
[36]
Elsayed and B.V
T.A. Elsayed and B.V. Fine, Regression relation for pure quantum states and its implications for efficient computing , Physical Review Letters 110 (2013) 070404
2013
-
[37]
Niemeijer, Some exact calculations on a chain of spins 1/2 , Physica 36 (1967)
T. Niemeijer, Some exact calculations on a chain of spins 1/2 , Physica 36 (1967)
1967
-
[38]
Richter, F
J. Richter, F. Jin, H. De Raedt, K. Michielsen, J. Gemmer and R. Steinigeweg, Real-time dynamics of typical and untypical states in nonintegrable systems , Phys. Rev. B 97 (2018) 174430
2018
-
[39]
Cullum and R.A
J.K. Cullum and R.A. Willoughby, Lanczos algorithms for large symmetric eigenvalue computations: Vol. I: Theory , SIAM (2002)
2002
-
[40]
Motta, C
M. Motta, C. Sun, A.T. Tan, M.J. O’Rourke, E. Ye, A.J. Minnich et al., Determining eigenstates and thermal states on a quantum computer using quantum imaginary time evolution, Nature Physics 16 (2020) 205
2020
-
[41]
Avdoshkin and A
A. Avdoshkin and A. Dymarsky, Euclidean operator growth and quantum chaos , Phys. Rev. Res. 2 (2020) 043234 [ 1911.09672]. – 33 –
2020 arXiv
Reviewed August 5, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.