Generalized Spherical Neural Operators: Green's Function Formulation
Pith reviewed 2026-05-16 23:02 UTC · model grok-4.3
The pith
A designable Green's function on the sphere lets neural operators flexibly mix rotational symmetry with real-world asymmetry while keeping spectral speed and grid freedom.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
A designable spherical Green's function together with its harmonic expansion supplies a solid operator-theoretic foundation for spherical learning; absolute and relative position-dependent versions of this kernel enable a tunable balance between equivariance and invariance; the resulting Green's-function Spherical Neural Operator (GSNO), equipped with a novel spectral learning method, adapts to non-equivariant systems while retaining spectral efficiency and grid invariance.
What carries the argument
The designable spherical Green's function and its harmonic expansion, which acts as the integral kernel of the neural operator and controls the equivariance-invariance trade-off through absolute and relative positional weighting.
If this is right
- GSNO can model systems that violate rotational symmetry without losing computational efficiency.
- The operator remains invariant to the choice of spherical grid discretization.
- SHNet's multi-scale spectral modeling plus spherical sampling improves global feature representation.
- Empirical results show outperformance on diffusion MRI, shallow-water dynamics, and global weather forecasting.
Where Pith is reading between the lines
- Similar Green's-function constructions could extend the approach to other curved manifolds beyond the sphere.
- The spectral learning procedure may reduce training cost for large-scale global simulation tasks.
- The framework offers a route to hybrid operators that default to equivariance but relax it only where data demands it.
Load-bearing premise
A designable spherical Green's function and its harmonic expansion can be constructed to balance equivariance and invariance without introducing unaccounted distortions in complex real-world systems.
What would settle it
A controlled spherical PDE experiment where the learned GSNO produces larger errors or visible grid artifacts than a strictly equivariant baseline on data that breaks rotational symmetry.
Figures
read the original abstract
Neural operators offer powerful approaches for solving parametric partial differential equations, but extending them to spherical domains remains challenging due to the need to preserve intrinsic geometry while avoiding distortions that break rotational consistency. Existing spherical operators rely on rotational equivariance but often lack the flexibility for real-world complexity. We propose a generalized operator-design framework based on the designable spherical Green's function and its harmonic expansion, establishing a solid operator-theoretic foundation for spherical learning. Based on this, we propose an absolute and relative position-dependent Green's function that enables flexible balance of equivariance and invariance for real-world modeling. The resulting operator, Green's-function Spherical Neural Operator (GSNO) with a novel spectral learning method, can adapt to non-equivariant systems while retaining spectral efficiency and grid invariance. To exploit GSNO, we develop SHNet, a hierarchical architecture that combines multi-scale spectral modeling with spherical up-down sampling, enhancing global feature representation. Evaluations on diffusion MRI, shallow water dynamics, and global weather forecasting, GSNO and SHNet consistently outperform state-of-the-art methods. The theoretical and experimental results position GSNO as a principled and generalized framework for spherical operator design and learning, bridging rigorous theory with real-world complexity. The code is available at: https://github.com/haot2025/GSNO.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a generalized framework for spherical neural operators grounded in a designable spherical Green's function and its harmonic expansion. It introduces the Green's-function Spherical Neural Operator (GSNO) with a novel spectral learning method based on absolute and relative position-dependent Green's functions to flexibly balance equivariance and invariance. The work also presents SHNet, a hierarchical architecture combining multi-scale spectral modeling with spherical up-down sampling. Evaluations on diffusion MRI, shallow water dynamics, and global weather forecasting claim consistent outperformance over state-of-the-art methods, with code released.
Significance. If the central claims hold, particularly that the position-dependent Green's function construction yields an operator that is simultaneously non-equivariant and exactly grid-invariant with spectral efficiency, the work would supply a principled operator-theoretic foundation for spherical learning. This could meaningfully advance applications in climate modeling, fluid dynamics, and medical imaging on spherical domains, especially given the code release for reproducibility.
major comments (2)
- [Theoretical development] Theoretical development (Green's function formulation): The claim that the harmonic expansion of the absolute/relative position-dependent Green's function yields an operator that retains exact grid invariance and spectral efficiency for non-equivariant systems is not supported by an explicit truncation-error bound or a proof that learned coefficients remain consistent across arbitrary discretizations; any finite truncation produces a kernel that is no longer exactly diagonal in the spherical-harmonic basis, directly threatening the central claim once rotational equivariance is relaxed.
- [Abstract and evaluations] Abstract and experimental sections: The outperformance statements on the three tasks are presented without reported error bars, ablation studies isolating the spectral learning method, or explicit comparisons against strong equivariant baselines, leaving open whether post-hoc design choices or data-specific factors drive the reported gains.
minor comments (2)
- [Abstract] The abstract refers to 'designable spherical Green's function' without a concise definition or reference to the precise functional form used in the construction.
- [Theoretical development] Notation for the absolute versus relative position dependence should be introduced with a single equation early in the theoretical section to avoid later ambiguity.
Simulated Author's Rebuttal
We thank the referee for the detailed and constructive report. We address each major comment below and outline the revisions we will make to strengthen the manuscript.
read point-by-point responses
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Referee: [Theoretical development] Theoretical development (Green's function formulation): The claim that the harmonic expansion of the absolute/relative position-dependent Green's function yields an operator that retains exact grid invariance and spectral efficiency for non-equivariant systems is not supported by an explicit truncation-error bound or a proof that learned coefficients remain consistent across arbitrary discretizations; any finite truncation produces a kernel that is no longer exactly diagonal in the spherical-harmonic basis, directly threatening the central claim once rotational equivariance is relaxed.
Authors: We agree that an explicit truncation-error bound would strengthen the theoretical claims. The manuscript presents the harmonic expansion as exact in the infinite-dimensional spherical-harmonic basis, with the position-dependent Green's function (absolute and relative) designed so that the learned operator remains diagonal in that basis by construction. Finite truncation is an approximation shared by all spectral methods; grid invariance holds exactly for the continuous operator and is preserved up to the discretization error of the chosen quadrature. To address the referee's concern directly, we will add a new subsection deriving a truncation-error bound that quantifies the deviation from exact diagonality as a function of the number of retained harmonics, together with a consistency argument showing that the learned coefficients converge to the same continuous operator under refinement of the discretization. This revision will be included in the next version. revision: yes
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Referee: [Abstract and evaluations] Abstract and experimental sections: The outperformance statements on the three tasks are presented without reported error bars, ablation studies isolating the spectral learning method, or explicit comparisons against strong equivariant baselines, leaving open whether post-hoc design choices or data-specific factors drive the reported gains.
Authors: We acknowledge that the current experimental section would benefit from additional statistical rigor and controls. In the revised manuscript we will: (i) report mean performance with standard error bars computed over at least five independent runs with different random seeds; (ii) add ablation studies that isolate the contribution of the absolute versus relative position-dependent Green's functions and the spectral learning procedure; and (iii) include direct comparisons against strong equivariant baselines (e.g., spherical CNNs and rotation-equivariant neural operators) on the same data splits. These additions will clarify the source of the observed improvements and will be placed in the experimental section with corresponding updates to the abstract. revision: yes
Circularity Check
No circularity: Green's function framework presented as independent design choice
full rationale
The provided abstract and description frame the spherical Green's function, its harmonic expansion, and the absolute/relative position-dependent variants as a proposed operator-theoretic foundation chosen by design. No equations, derivations, or steps are exhibited that reduce any prediction or result to a fitted parameter, self-citation chain, or definitional tautology. The spectral learning method and SHNet architecture are introduced as novel extensions without visible reduction to the inputs by construction. This matches the default expectation that most papers are non-circular; the central claims rest on the design choice itself rather than any self-referential loop.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Spherical Green's functions can be designed to preserve intrinsic geometry and support flexible equivariance-invariance balance
invented entities (1)
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Absolute and relative position-dependent Green's function
no independent evidence
Lean theorems connected to this paper
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IndisputableMonolith/Foundation/AlexanderDuality.leanalexander_duality_circle_linking unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We propose a generalized operator-design framework based on the designable spherical Green's function and its harmonic expansion... G(R^{-1}u) and its spherical harmonic extension... I(T_orig) = 2π√(4π/(2l+1))·SHT[G1](l,0)·SHT[f](l,m)
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
The resulting operator... can adapt to non-equivariant systems while retaining spectral efficiency and grid invariance
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
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14 Published as a conference paper at ICLR 2026 A SPHERICALCONVOLUTIONTHEOREM A.1 PROPERTIES OF SPHERICAL HARMONICS The explicit form of spherical harmonic function (Müller,
work page 2026
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[25]
is: Y m l (θ, ϕ) = s (2l+ 1)(l−m)! 4π(l+m)! P m l (cosθ)e imϕ (21) Spherical harmonic functions satisfy the orthonormality condition (Müller, 2006): Z S2 Y m l (ω)Y m′ l′ (ω)dω=δ ll′δmm′.(22) whereδis the Kronecker delta. For subsequent analysis, we recall how the spherical harmonics transform under a rotation operation RfromSO(3). The rotation of the sph...
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[26]
andΛ(R)represents the rotation oper- ator acting on spherical functions. Taking the complex conjugate of Equation equation 23 yields: Y m l (Rω) = X |k|≤l D(l) k,m(R−1)Y k l (ω).(24) A.2 INTERCHANGE OF INTEGRALS AND VARIABLE SUBSTITUTION Interchange integral order after substituting the convolution definition into the spherical harmonic transform: SHT[(f∗...
work page 1994
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[27]
Specifically, the encoder first expands the input channel toC
Therefore,only thek= 0term survives.So the overall integral is simplified to: SHT[(f∗h)](l, m) =SHT[h](l,0) Z SO(3) f(Rn) D(l) 0,m(R−1)dR.(32) A.4 RELATIONSHIP BETWEEND-MATRICES AND SPHERICAL HARMONICS Because the unitary characteristic of the Wigner D-matrix (Driscoll & Healy, 1994), we can get the relationship between D-matrix and spherical Harmonic fun...
work page 1994
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[28]
Our proposed SHNet with a depth of 2 consists of five blocks, including two downsampling layers, two upsampling layers, and an output layer. Furthermore, the embedding dimension of SHNet is set to a smaller value (8 and 64), as the depth increases, the embedding dimension gradually increases until 17 Published as a conference paper at ICLR 2026 it is four...
work page 2026
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[29]
This loss function is used in all the experiments
as follows: L[F(X n), Yn+t] = 1 C CX c=1 P i,j vi,j|F(X n)(xc,i,j)−Y n+t(xc,i,j)|2 P i,j vi,j|Yn+t(xc,i,j)|2 ! 1 2 ,(37) wherev i,j is the products of the Jacobiansin(λ i)(λ i represents the latitude at grid point) and the quadrature weights (Bonev et al., 2023).Cis the number of predicted variables.nis the index of the initial time step,tis the predicted...
work page 2023
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[30]
C.3 WEATHER FORECASTING The dataset is publicly available at WeatherBench GitHub (Rasp et al., 2020). Following (Liu et al., 2024), the chosen 24 common variables are detailed in Table 9, including 10U, 10V , 2T, U50, U50, U250, U500, U600, U700, U850, U925, V50, V250, V500, V600, V700, V850, V925, T50, T250, T500, T600, T700, T850, T925. And 10U, 10V , 2...
work page 2020
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[31]
as: MSE[F(X n), Yn+t] = 1 C×H×W CX c=1 HX i=1 WX j=1 wi (F(X n)(xc,i,j)−Y n+t(xc,i,j))2 (38) 18 Published as a conference paper at ICLR 2026 Table 6: Supplementary experiments on SSWE at128×256resolution (MRE↓) and WB at64×128 resolution (ACC↑). Methods SSWE at 5h SSWE at 10h WB at 1day WB at 3day WB at 5day FNO2.22 2.73 93.4 71.5 42.5 SFNO0.74 0.87 91.6 ...
work page 2026
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[32]
Spatial position-embedding SNO
is used to measure spatial correlation between predicted anomalies F(X n)′ and target anomaliesY ′ n+t: ACC= P c,i,j wi F(X n)′ c,i,j ·(Y n+t)′ c,i,j qP c,i,j wi F(X n)′ c,i,j 2 ·P c,i,j wi (Yn+t)′ c,i,j 2 (40) Besides, to empirically compare the correction term of GSNO with positional embedding, we con- struct a "Spatial position-embedding SNO" model tha...
work page 2013
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[33]
on the single-shell LARDI to obtain single-shell low angular resolution FOD as the condition of GSNO (LAR-FOD).l max is set to the default value of 8 to balance preci- sion and complexity (Zeng et al., 2022). SSMT-CSD (Khan et al., 2020), FOD-Net (Zeng et al., 2022), FOD-SFNO (Bonev et al., 2023), ESCNN (Snoussi & Karimi,
work page 2022
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[34]
20 Published as a conference paper at ICLR 2026 Table 10: Parameters on dMRI. Models FOD-Net FOD-SFNO ESCNN FOD-GSNO Parameters 19.44 M 1.15 M 1.47 M 1.21 M Figure 6: Predicted temperature field at 2m from the earth surface of GSNO w/o I(T orig). Provide the residual (error) to the ground truth for clarity (darker is better). C.5 EXTRAEXPERIMENTS ANDINTER...
work page 2026
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[35]
As shown in Figure 6, even if I(T orig)term is frozen, using only I(T corr) term clearly outlines the fundamental temperature distribution framework linked to the Earth’s to- pography, successfully distinguishing the persistent low temperatures at the poles from the overall high-temperature pattern in regions like the equator. This explicitly demonstrates...
work page 2026
discussion (0)
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