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SYK Bilocal Collective Fields as Near-AdS2 Data

At large NN, the quartic Sachdev–Ye–Kitaev (SYK) model replaces NN interacting fermions by two functions of two Euclidean times. GG records the collective two-point function, while Σ\Sigma enforces its definition and becomes its self-energy at the saddle. Their stationary equations are closed and can be solved numerically.

This page derives that replacement, computes a finite-temperature q=4q=4 benchmark, and states exactly which disorder average and order of limits the answer represents. The resulting saddle and fluctuation kernel are useful near-AdS₂ comparison data. They are not local bulk fields, a unique microscopic Hamiltonian, or evidence for a complete gravity dual.

Required background. SYK Models and Local Quantum Criticality fixes the Majorana normalization, disorder variance, and no-minus Green-function convention used below.

Helpful background. Vector Models, Auxiliary Fields, and Large-N Saddles develops collective-field saddle methods; Replica and Supersymmetry Methods for Disorder separates exact averaging, analytic continuation, and saddle assumptions; Near-AdS₂, SYK, and the Duality Interface explains what the gravity comparison does and does not identify.

Scope. We study the standard even-NN, Gaussian, quartic Majorana ensemble on a Euclidean thermal circle. The explicit saddle calculation is the one-copy annealed problem at leading order in NN. Replica and fixed-realization statements are labeled separately.

Evidence cutoff: 29 August 2026.

The quartic ensemble and its thermal circle

Section titled “The quartic ensemble and its thermal circle”

Let NN be even and normalize the Majoranas by

{χi,χj}=δij,χi2=12.\{\chi_i,\chi_j\}=\delta_{ij}, \qquad \chi_i^2=\frac12.

For q=4q=4, take

HJ=∑1≤i<j<k<l≤NJijklχiχjχkχl,H_J=\sum_{1\le i<j<k<l\le N} J_{ijkl}\chi_i\chi_j\chi_k\chi_l,

with independent Gaussian couplings

Jijkl‾=0,Jijkl2‾=3!J2N3.\overline{J_{ijkl}}=0, \qquad \overline{J_{ijkl}^{2}}=\frac{3!J^2}{N^3}.

The frequently used general-qq phase iq/2i^{q/2} changes only an overall sign at q=4q=4; the symmetric Gaussian distribution absorbs that sign. The displayed convention therefore agrees with the prerequisite page.

Define the no-minus Euclidean correlator

GJ(τ1,τ2)=1N∑i=1N⟨Tτχi(τ1)χi(τ2)⟩J.G_J(\tau_1,\tau_2) =\frac1N\sum_{i=1}^N \left\langle T_\tau\chi_i(\tau_1)\chi_i(\tau_2)\right\rangle_J.

It is antisymmetric in its two arguments. At a translation-invariant thermal saddle, write G(τ1,τ2)=G(τ1−τ2)G(\tau_1,\tau_2)=G(\tau_1-\tau_2) and use

G(iωn)=∫0βdτ eiωnτG(τ),G(τ)=1β∑n∈Ze−iωnτG(iωn),ωn=(2n+1)πβ.\begin{aligned} G(i\omega_n) &=\int_0^\beta d\tau\, e^{i\omega_n\tau}G(\tau),\\ G(\tau) &=\frac1\beta\sum_{n\in\mathbb Z} e^{-i\omega_n\tau}G(i\omega_n),\\ \omega_n&=\frac{(2n+1)\pi}{\beta}. \end{aligned}

Before doing any calculation, distinguish the operations often compressed into the phrase “the SYK average.”

Which quantity is being computed?
Quantity or ansatz Operation Collective-field route Strongest immediate statement
Fixed realization ZJ Keep every coupling fixed Not produced by disorder averaging One sample-specific quantum system
Annealed log partition function Average ZJ, then take the logarithm The one-copy action derived below Annealed thermodynamics of the declared ensemble
Quenched log partition function Take log ZJ, then average Integer replicas followed by a formal n → 0 continuation Quenched thermodynamics, if the continuation and saddle are controlled
Replica-diagonal saddle Set off-diagonal saddle fields to zero An ansatz inside the replicated theory A candidate saddle, not a proof of dominance
Replica-nondiagonal sector Allow Gab and Σab for a ≠ b Fluctuations and possibly additional saddles Existence, stability, dominance, and the n → 0 limit are separate tests

Disorder contraction produces the G–Σ action

Section titled “Disorder contraction produces the G–Σ action”

For one fixed coupling realization,

ZJ=∫Dχ exp⁡ ⁣[−12∑i∫0βdτ χi∂τχi−∫0βdτ HJ(τ)].Z_J=\int\mathcal D\chi\, \exp\!\left[ -\frac12\sum_i\int_0^\beta d\tau\, \chi_i\partial_\tau\chi_i -\int_0^\beta d\tau\,H_J(\tau) \right].

The couplings are Gaussian, so their average can be done exactly. Define the off-shell fermion bilinear

Gχ(τ1,τ2)=1N∑iχi(τ1)χi(τ2).G_\chi(\tau_1,\tau_2) =\frac1N\sum_i \chi_i(\tau_1)\chi_i(\tau_2).

The O(N4)O(N^4) index choices combine with the variance O(N−3)O(N^{-3}) to leave an O(N)O(N) interaction:

ZJ‾=∫Dχ exp⁡ ⁣[−12∫χ∂τχ+NJ28∫dτ1dτ2 Gχ(τ1,τ2)4].\overline{Z_J} =\int\mathcal D\chi\, \exp\!\left[ -\frac12\int\chi\partial_\tau\chi +\frac{NJ^2}{8} \int d\tau_1d\tau_2\,G_\chi(\tau_1,\tau_2)^4 \right].

Insert a functional delta constraint that replaces GχG_\chi by an independent antisymmetric field GG and introduces an antisymmetric multiplier Σ\Sigma. Integrating the NN Majoranas gives a Pfaffian:

ZJ‾=N∫DG DΣ e−NI[G,Σ],\overline{Z_J} =\mathcal N\int\mathcal DG\,\mathcal D\Sigma\, e^{-N\mathcal I[G,\Sigma]},

with

I[G,Σ]=−log⁡Pf⁡(∂τ−Σ)+12∫dτ1dτ2[ΣG−J24G4].\boxed{ \mathcal I[G,\Sigma] =-\log\operatorname{Pf}(\partial_\tau-\Sigma) +\frac12\int d\tau_1d\tau_2 \left[ \Sigma G-\frac{J^2}{4}G^4 \right]. }

This is equivalently

−log⁡Pf⁡(∂τ−Σ)=−12Tr⁡log⁡(∂τ−Σ).-\log\operatorname{Pf}(\partial_\tau-\Sigma) =-\frac12\operatorname{Tr} \log(\partial_\tau-\Sigma).

It is not −12log⁡Pf⁡-\tfrac12\log\operatorname{Pf}: that expression would be −14Tr⁡log⁡-\tfrac14\operatorname{Tr}\log and would miss a factor of two in the first saddle equation. The correctly normalized Pfaffian action appears in Maldacena and Stanford 2016, § 2.6, Eqs. (2.25)–(2.26).

The multiplier contour initially enforces the delta functional and is then deformed through an appropriate saddle. The normalization N\mathcal N, contour choice, regulator, and functional Jacobian do not change the leading O(N)O(N) stationary equations, but they matter for the O(N0)O(N^0) fluctuation determinant. Thus the displayed action is a leading large-NN object, not a complete finite-NN measure.

For an antisymmetric operator AA,

δlog⁡Pf⁡A=12Tr⁡(A−1δA).\delta\log\operatorname{Pf}A =\frac12\operatorname{Tr}(A^{-1}\delta A).

Take A=∂τ−ΣA=\partial_\tau-\Sigma. In the trace, the kernel order is reversed; antisymmetry then converts the variation of the Pfaffian term into −12∫G δΣ-\tfrac12\int G\,\delta\Sigma. It cancels the explicit +12∫G δΣ+\tfrac12\int G\,\delta\Sigma precisely when

G=(∂τ−Σ)−1.G=(\partial_\tau-\Sigma)^{-1}.

Variation with respect to GG gives the second equation. Together,

(∂τ−Σ)∘G=δ,Σ(τ1,τ2)=J2G(τ1,τ2)3,\begin{aligned} (\partial_\tau-\Sigma)\circ G&=\delta,\\ \Sigma(\tau_1,\tau_2)&=J^2G(\tau_1,\tau_2)^3, \end{aligned}

where

(A∘B)(τ1,τ3)=∫0βdτ2 A(τ1,τ2)B(τ2,τ3).(A\circ B)(\tau_1,\tau_3) =\int_0^\beta d\tau_2\, A(\tau_1,\tau_2)B(\tau_2,\tau_3).

At a translation-invariant saddle these become

G(iωn)=1−iωn−Σ(iωn),Σ(τ)=J2G(τ)3.\begin{aligned} G(i\omega_n) &=\frac1{-i\omega_n-\Sigma(i\omega_n)},\\ \Sigma(\tau)&=J^2G(\tau)^3. \end{aligned}

Four elementary checks catch most sign and normalization errors:

  • Fermion exchange: G(τ1,τ2)=−G(τ2,τ1)G(\tau_1,\tau_2)=-G(\tau_2,\tau_1).
  • Thermal reflection: antiperiodicity implies G(β−τ)=G(τ)G(\beta-\tau)=G(\tau) for 0<τ<β0<\tau<\beta.
  • Equal-time jump: G(0+)=12G(0^+)=\tfrac12 and G(0−)=−12G(0^-)=-\tfrac12 in the no-minus convention.
  • Ultraviolet tail: G(iωn)=(−iωn)−1+O(ωn−2)G(i\omega_n)=(-i\omega_n)^{-1}+O(\omega_n^{-2}).

These equations and checks are reviewed with the corresponding diagrammatics in Chowdhury et al. 2022, §§ II.B–II.D.

Set J=1J=1. For each βJ∈{5,10,20,50}\beta J\in\{5,10,20,50\}, solve the full equations rather than dropping the derivative term. Some references instead use J=q J/2(q−1)/2\mathcal J=\sqrt q\,J/2^{(q-1)/2}; at q=4q=4, J=J/2\mathcal J=J/\sqrt2. Every temperature in the table is expressed using JJ.

The calculation uses

τj=(j+12)βM,n=−M2,…,M2−1,\tau_j=\frac{(j+\tfrac12)\beta}{M}, \qquad n=-\frac M2,\ldots,\frac M2-1,

with the matching finite Fourier pair. Start from G0(iωn)=(−iωn)−1G_0(i\omega_n)=(-i\omega_n)^{-1}. Before transforming to time, subtract this known ultraviolet tail and add its exact value G0(τ)=1/2G_0(\tau)=1/2 for 0<τ<β0<\tau<\beta. One iteration is

G(iωn)⟶G(τ)⟶Σ(τ)⟶Σ(iωn)⟶G~(iωn),G(i\omega_n) \longrightarrow G(\tau) \longrightarrow \Sigma(\tau) \longrightarrow \Sigma(i\omega_n) \longrightarrow \widetilde G(i\omega_n),

followed by

Gk+1=(1−x)Gk+xG~k,x=0.25.G_{k+1}=(1-x)G_k+x\widetilde G_k, \qquad x=0.25.

Stop when the relative fixed-point update is below 10−1310^{-13}. Independently report the Dyson residual

RSD=max⁡n∣[−iωn−Σ(iωn)]G(iωn)−1∣.R_{\mathrm{SD}} =\max_n\left\lvert \left[-i\omega_n-\Sigma(i\omega_n)\right] G(i\omega_n)-1 \right\rvert.

The primary grid has M=1024M=1024 points. A second calculation with M=2048M=2048 supplies a discretization estimate. For comparison, the thermal conformal solution for 0<τ<β0<\tau<\beta is

Gc(τ)=b[πβJsin⁡(πτ/β)]1/2,b4=14π.G_c(\tau) =b\left[ \frac{\pi}{\beta J\sin(\pi\tau/\beta)} \right]^{1/2}, \qquad b^4=\frac1{4\pi}.
Finite-temperature q = 4 saddle, J = 1 and M = 2048
βJ G(β/4) G(β/2) Gc(β/2) Midpoint difference from conformal 1024 → 2048 midpoint shift RSD
5 0.391691 0.364121 0.421005 −13.51% 2.26 × 10−8 1.8 × 10−14
10 0.313992 0.279188 0.297696 −6.22% 3.02 × 10−8 3.5 × 10−14
20 0.236904 0.204449 0.210503 −2.88% 3.50 × 10−8 5.0 × 10−14
50 0.155164 0.131678 0.133134 −1.09% 3.02 × 10−8 9.3 × 10−14

The four runs converged in 77–82 iterations. On the M=1024M=1024 grid, Fourier closure was better than 4.4×10−144.4\times10^{-14}, reflection symmetry better than 1.3×10−151.3\times10^{-15}, and imaginary leakage below 2.9×10−162.9\times10^{-16}. The median ultraviolet-tail error over the highest one-eighth of Matsubara modes ranged from 1.0×10−61.0\times10^{-6} at βJ=5\beta J=5 to 1.1×10−41.1\times10^{-4} at βJ=50\beta J=50.

The largest last-grid shift, 3.6×10−83.6\times10^{-8} in G(β/2)G(\beta/2), is a discretization estimate rather than a rigorous error bound. It dominates the iteration error but is far smaller than the physical difference from the conformal approximation. The systematic trend is clear: the full saddle approaches the conformal answer as βJ\beta J grows, while the short-time endpoints remain ultraviolet-sensitive. The algorithm follows Maldacena and Stanford 2016, Appendix G, with every grid, tail, mixing, stopping, and comparison choice stated here.

Bilocal fluctuations organize four-point data

Section titled “Bilocal fluctuations organize four-point data”

Write fluctuations around the saddle as

G=G⋆+N−1/2g,Σ=Σ⋆+N−1/2σ.G=G_\star+N^{-1/2}g, \qquad \Sigma=\Sigma_\star+N^{-1/2}\sigma.

The linear term vanishes by stationarity. The quadratic O(N0)O(N^0) action couples gg and σ\sigma. Eliminating σ\sigma produces the ladder kernel; in index-free notation its action contains the operator 1−K1-K, with

K(τ1,τ2;τ3,τ4)=−3J2G⋆(τ13)G⋆(τ24)G⋆(τ34)2K(\tau_1,\tau_2;\tau_3,\tau_4) =-3J^2 G_\star(\tau_{13})G_\star(\tau_{24}) G_\star(\tau_{34})^2

for q=4q=4 in this convention. The inverse (1−K)−1(1-K)^{-1} sums ladder diagrams and gives the connected four-point function. A nearly unit eigenvalue in the reparametrization channel is the precursor of the Schwarzian soft mode. The derivation and its normalization are given in Maldacena and Stanford 2016, §§ 3.1 and 4; the bilocal fluctuation viewpoint is developed in Jevicki, Suzuki, and Yoon 2016, §§ 2–3.

The pair (τ1,τ2)(\tau_1,\tau_2) can be reorganized into a center time and a relative time, which makes some conformal structures resemble a field in one extra dimension. That is a useful kinematic representation, not a proof of locality: the Pfaffian and the full quadratic kernel remain nonlocal in the bilocal coordinates. The next page, SYK Conformal Regime and Schwarzian Matching, extracts the soft sector without identifying every bilocal fluctuation with a local bulk particle.

Three infrared boundaries are physically different

Section titled “Three infrared boundaries are physically different”

The shorthand “finite-NN cutoff” hides distinct mechanisms. For a low-energy state with entropy S(E)=O(N)S(E)=O(N), the relevant hierarchy is parametrically

J≫JN≫δE(E),δE(E)∼Je−S(E),J \gg \frac JN \gg \delta E(E), \qquad \delta E(E)\sim J e^{-S(E)},

up to model- and energy-dependent prefactors.

Do not collapse the three boundaries into one cutoff
Boundary Parametric scale What changes What remains useful
Ultraviolet crossover Energy of order J The derivative term cannot be dropped The full bilocal Schwinger–Dyson equations
Quantum Schwarzian crossover Energy parametrically of order J/N The classical conformal saddle is insufficient; the soft mode fluctuates strongly The Schwarzian effective theory in its stated window
Sample-specific discreteness Mean level spacing of order J e−S(E) A smooth density cannot resolve individual levels or recurrences Coarse or averaged spectral observables above the spacing scale

Thus J/NJ/N is not the many-body level spacing. The classical conformal window may be summarized as J/N≪max⁡(T,∣ω∣)≪JJ/N\ll\max(T,|\omega|)\ll J only when one is specifically discussing the leading conformal saddle. Below J/NJ/N, the Schwarzian sector is the next effective description; exponentially farther into the infrared, a fixed sample’s discrete spectrum becomes unavoidable.

Quenched and annealed claims need separate evidence

Section titled “Quenched and annealed claims need separate evidence”

The one-copy derivation computed ZJ‾\overline{Z_J}. The quenched quantity begins instead with an integer replica number:

ZJn‾=Nn∫DGab DΣab e−NIn[G,Σ],\overline{Z_J^n} =\mathcal N_n \int\mathcal DG_{ab}\,\mathcal D\Sigma_{ab}\, e^{-N\mathcal I_n[G,\Sigma]},

where

In=−log⁡Pf⁡(∂τδab−Σab)+12∑a,b=1n∫dτ1dτ2[ΣabGab−J24Gab4].\begin{aligned} \mathcal I_n ={}&-\log\operatorname{Pf} \left(\partial_\tau\delta_{ab}-\Sigma_{ab}\right)\\ &+\frac12\sum_{a,b=1}^n \int d\tau_1d\tau_2 \left[ \Sigma_{ab}G_{ab} -\frac{J^2}{4}G_{ab}^{4} \right]. \end{aligned}

Here Gab(τ1,τ2)=−Gba(τ2,τ1)G_{ab}(\tau_1,\tau_2)=-G_{ba}(\tau_2,\tau_1), and likewise for Σab\Sigma_{ab}. The quenched free energy is formally

Fq=−1βlog⁡ZJ‾=−1βlim⁡n→0ZJn‾−1n.F_{\mathrm q} =-\frac1\beta\overline{\log Z_J} =-\frac1\beta \lim_{n\to0} \frac{\overline{Z_J^n}-1}{n}.

The replica-diagonal choice Gab=δabGG_{ab}=\delta_{ab}G and Σab=δabΣ\Sigma_{ab}=\delta_{ab}\Sigma reduces the saddle to nn copies of the one-copy equations. Calling this an ansatz keeps the logic honest: dominance, analytic continuation, and the absence of another saddle are additional questions.

At every finite NN, Jensen’s inequality gives

log⁡ZJ‾≤log⁡ZJ‾,Fq≥Fa.\overline{\log Z_J} \le \log\overline{Z_J}, \qquad F_{\mathrm q}\ge F_{\mathrm a}.

Equality of leading free-energy densities is nevertheless much better supported for the standard fermionic ensemble than an appeal to replica diagonality alone:

  • In the normalization studied by Anschuetz, Chen, Kiani, and King, the intensive annealed–quenched log-partition gap is bounded by Oq(β2N−q/2)O_q(\beta^2N^{-q/2}). It therefore vanishes for β=o(Nq/4)\beta=o(N^{q/4})—for q=4q=4, β=o(N)\beta=o(N)—but the result does not reach exponentially low temperatures or identify finite-NN samples. Constants must be translated before combining their normalization with the JJ convention on this page Anschuetz et al. 2025, § IV, Eq. (9), and Appendix C, Corollary C.1.
  • For every fixed even qq, Gamarnik, Pernice, Schmidhuber, and Zlokapa prove the existence and compute the common annealed and quenched free-energy density at sufficiently small but constant β\beta for their Rademacher-coupling ensemble and normalized trace. Their q=4q=4 numerical comparison agrees with the physics bilocal equations over the tested interval β∈[0,3]\beta\in[0,3]; that interval is not itself the theorem’s full rigorously established range. Applying the theorem literally to the Gaussian JJ convention above requires a separate universality and normalization translation Gamarnik et al. 2026, § 2, Theorem 5, and § 5, Eqs. (40)–(41) and Figure 3.
  • Replica-nondiagonal solutions do exist in controlled ansätze. Aref’eva and collaborators found finite-integer-replica candidates that are subleading and singular as n→0n\to0, alongside strong-coupling conformal candidates whose regulated continuation requires separate interpretation Aref’eva et al. 2019, §§ 3–5 and Appendix B.
  • Numerical long-time correlators support the replica-diagonal saddle plus Gaussian fluctuations—including off-diagonal fluctuation components—on times shorter than the inverse many-body level spacing Wang et al. 2019, §§ 3–5. Independent analytic and spectral tests found no spin-glass phase in the standard model within their examined regimes Gur-Ari, Mahajan, and Vaezi 2018, §§ 2–3.

These statements concern the standard fermionic SYK ensemble. Bosonic or spin variants can behave differently; the comparison in Baldwin and Swingle 2020, §§ II–IV is a warning against transferring the conclusion by name alone.

Adversarial control: change the averaging hypothesis

Section titled “Adversarial control: change the averaging hypothesis”

Now change one assumption at a time.

Surviving claim under an adversarial change of average
Change What can survive What no longer follows Reason for the downgrade
Annealed → quenched Leading free-energy density and coarse correlators in regimes covered by concentration or a controlled saddle Finite-N equality or sample-specific spectral data The logarithm and disorder average do not commute
Add off-diagonal Gaussian fluctuations Replica-diagonal leading saddle and corrected four-point functions A strictly diagonal fluctuation theory Off-diagonal fluctuations need not imply an off-diagonal saddle
Admit a dominant nondiagonal saddle The exact replicated functional integral The one-copy saddle as the full quenched answer The saddle and possibly its n → 0 continuation have changed
Ensemble → one fixed realization Self-averaging coarse observables in a demonstrated regime Exact late-time recurrences, individual levels, or a unique nonperturbative completion The smooth collective saddle has discarded realization-specific information

The strongest claim that survives all of these tests is deliberately limited: a normalized large-NN SYK saddle, its fluctuation kernel, and their averaging convention can supply coarse infrared data that match selected near-AdS₂ observables. Annealed–quenched agreement of an intensive quantity does not turn an ensemble into one Hamiltonian; replica-nondiagonal saddle existence does not establish dominance; and melonic bilocal dynamics does not by itself prove a gravity dual.

Halving the Pfaffian twice. A Majorana Gaussian integral gives a Pfaffian, and log⁡Pf⁡A=12Tr⁡log⁡A\log\operatorname{Pf}A=\tfrac12\operatorname{Tr}\log A. Writing both the Pfaffian and an additional factor 1/21/2 loses a factor of two.

Calling the one-copy action replica diagonal. The one-copy action has no replica indices and computes an annealed object. “Replica diagonal” describes an ansatz made only after constructing Zn‾\overline{Z^n}.

Equating existence with dominance. A stationary point can be subleading, unstable, or singular under n→0n\to0. Its action, Hessian, contour, and continuation must be checked.

Using 1/N1/N as a level spacing. The scale J/NJ/N governs the quantum soft-mode crossover. A typical many-body level spacing is exponentially smaller, of order Je−S(E)Je^{-S(E)}.

Turning bilocal coordinates into a local bulk. Reorganizing two times as center and relative coordinates is kinematically useful. It does not remove the nonlocal Pfaffian or prove a local two-dimensional bulk action.

Show why replacing −log⁡Pf⁡A-\log\operatorname{Pf}A by −12log⁡Pf⁡A-\tfrac12\log\operatorname{Pf}A changes the saddle normalization.

Solution

For antisymmetric AA,

Pf⁡(A)2=det⁡A,\operatorname{Pf}(A)^2=\det A,

so

−log⁡Pf⁡A=−12Tr⁡log⁡A.-\log\operatorname{Pf}A =-\frac12\operatorname{Tr}\log A.

An extra factor 1/21/2 would instead give −14Tr⁡log⁡A-\tfrac14\operatorname{Tr}\log A. Its variation would be half as large, while the explicit ΣG\Sigma G term would be unchanged. The resulting stationarity condition could not be G=A−1G=A^{-1} with the displayed interaction normalization.

Derive both Schwinger–Dyson equations from I[G,Σ]\mathcal I[G,\Sigma].

Solution

Use

δlog⁡Pf⁡A=12Tr⁡(A−1δA),A=∂τ−Σ.\delta\log\operatorname{Pf}A =\frac12\operatorname{Tr}(A^{-1}\delta A), \qquad A=\partial_\tau-\Sigma.

Keeping the reversed kernel order inside the trace and using antisymmetry, the Σ\Sigma variation gives

G=(∂τ−Σ)−1.G=(\partial_\tau-\Sigma)^{-1}.

The GG variation is

12∫dτ1dτ2 (Σ−J2G3)δG,\frac12\int d\tau_1d\tau_2\, \left(\Sigma-J^2G^3\right)\delta G,

so Σ=J2G3\Sigma=J^2G^3. Multiplying the first equation by ∂τ−Σ\partial_\tau-\Sigma gives the convolution form.

Find G(0±)G(0^\pm) and infer the leading large-frequency behavior.

Solution

The no-minus convention and χi2=1/2\chi_i^2=1/2 give

G(0+)=12.G(0^+)=\frac12.

Fermionic time ordering exchanges the operators across the origin, so G(0−)=−1/2G(0^-)=-1/2. The unit discontinuity is the Green function of the first-order kinetic operator, hence

G(iωn)∼1−iωn.G(i\omega_n)\sim\frac1{-i\omega_n}.

This is also obtained directly by neglecting Σ(iωn)\Sigma(i\omega_n) relative to ωn\omega_n at large frequency.

At βJ=50\beta J=50, compare the M=2048M=2048 midpoint value with the conformal value. Which numerical uncertainty dominates?

Solution

The fractional difference is

0.131678−0.1331340.133134≃−1.09%.\frac{0.131678-0.133134}{0.133134} \simeq-1.09\%.

The last grid shift is 3.02×10−83.02\times10^{-8}, whereas the Dyson residual is 9.3×10−149.3\times10^{-14}. Discretization therefore dominates iteration error. The 1.09% physical difference from the conformal approximation is still much larger than either numerical diagnostic.

5. Jensen’s inequality and free energies

Section titled “5. Jensen’s inequality and free energies”

Determine the ordering of annealed and quenched free energies.

Solution

Because log⁡\log is concave,

log⁡ZJ‾≤log⁡ZJ‾.\overline{\log Z_J}\le\log\overline{Z_J}.

Multiplying by −1/β-1/\beta reverses the inequality:

Fq≥Fa.F_{\mathrm q}\ge F_{\mathrm a}.

If their difference divided by NN vanishes in a large-NN regime, the leading free-energy densities agree there. The finite-NN operations remain distinct.

Suppose N=100N=100 and a relevant energy window has entropy S(E)=0.2NS(E)=0.2N. Compare J/NJ/N with the rough level-spacing estimate Je−S(E)Je^{-S(E)}.

Solution

The soft-mode scale is

JN=10−2J.\frac JN=10^{-2}J.

The spacing estimate is

Je−S(E)=Je−20≃2.1×10−9J.Je^{-S(E)}=Je^{-20}\simeq2.1\times10^{-9}J.

They differ by almost seven orders of magnitude. Crossing J/NJ/N calls for quantum Schwarzian dynamics; resolving Je−S(E)Je^{-S(E)} calls for the individual spectrum of a fixed finite-NN sample.

The chapter overview contains the structure diagram and validity and failure diagram. They are embedded there once so that their shared chapter-level context is not repeated on every article.

For the chapter-wide comparison of assumptions, counterevidence, falsifiers, and claim ceilings, see the claim-domain table.

  • Anschuetz, Eric R., Chi-Fang Chen, Bobak T. Kiani, and Robbie King. “Strongly Interacting Fermions Are Non-Trivial Yet Non-Glassy.” Physical Review Letters 135, 030602 (2025). DOI and open article. Open PDF.
  • Aref’eva, Irina, Mikhail Khramtsov, Maria Tikhanovskaya, and Igor Volovich. “Replica-Nondiagonal Solutions in the SYK Model.” Journal of High Energy Physics 2019, 7 (2019): 113. DOI and open article. Open PDF.
  • Baldwin, Christopher L., and Brian Swingle. “Quenched versus Annealed: Glassiness from SK to SYK.” Physical Review X 10, 031026 (2020). DOI and open article. Open PDF.
  • Chowdhury, Debanjan, Antoine Georges, Olivier Parcollet, and Subir Sachdev. “Sachdev–Ye–Kitaev Models and Beyond: A Window into Non-Fermi Liquids.” Reviews of Modern Physics 94, 035004 (2022). DOI. Open PDF.
  • Gamarnik, David, Francisco Pernice, Alexander Schmidhuber, and Alexander Zlokapa. “The Free Energy Limit of the SYK Model at High Temperature.” arXiv:2605.02768 [cond-mat.dis-nn, hep-th, math.PR, quant-ph] (2026). Open PDF.
  • Gur-Ari, Guy, Raghu Mahajan, and Abolhassan Vaezi. “Does the SYK Model Have a Spin Glass Phase?” Journal of High Energy Physics 2018, 11 (2018): 070. DOI and open article. Open PDF.
  • Jevicki, Antal, Kenta Suzuki, and Junggi Yoon. “Bi-Local Holography in the SYK Model.” Journal of High Energy Physics 2016, 7 (2016): 007. DOI and open article. Open PDF.
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