REVIEW 6 major objections 5 minor 30 references
The optimal Pad\'{e} polynomial for reconstruction of luminosity distance based on 10-fold cross-validation
T0 review · 6 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read The paper claims that a 10-fold cross-validation scheme selects the (2,1) Padé rational polynomial as the optimal reconstructor of luminosity distance from Pantheon+ supernova data.
desk verdict A well-intentioned but statistically underspecified attempt to make Padé order selection data-driven; the conclusion is plausible, but the procedure's reliability is not demonstrated. 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 Padé rational approximation $P_{m,n}(z) = (a_0+a_1 z+\dots+a_m z^m)/(1+b_1 z+\dots+b_n z^n)$ for luminosity distance as a function of redshift, with coefficients matched to the Taylor expansion of the cosmographic distance series. Around that object the paper builds a 10-fold cross-validation model-selection pipeline: random splitting, training each candidate order on $k-1$ folds, scoring held-out predictions by MSE, AIC, and BIC, and averaging over folds. AIC and BIC penalize the number of free coefficients, which is what lets the procedure trade fit quality against polynomial complexity. The Pantheon+ supernova sample supplies the observed distance moduli that the candidate rational forms must predict.
What would settle it
Recompute the 10-fold cross-validation scores for the four candidate Padé orders with a fully specified Gaussian likelihood using the Pantheon+ covariance matrix, repeat over many random data splits, and check whether the AIC/BIC gap between (2,1) and (2,2) or (3,2) exceeds the spread of scores across splits; if it does not, the claim that (2,1) is optimal is not supported by the cross-validation evidence.
Extended reading notes
Core claim
The central claim is that the (2,1) Padé rational polynomial is the optimal functional form for reconstructing luminosity distance from Pantheon+ data, and that this can be established by cross-validation rather than by theoretical priors. Using 2-, 5-, 10-, 20-, and 50-fold validation, the authors first screen Padé orders by mean squared error below 0.05, retain orders (2,1), (2,2), (3,1), and (3,2), and then compare fold-averaged MSE, AIC, and BIC. In 10-fold cross-validation the (2,1) order scores best on all three criteria, and MCMC fitting of its free parameters gives $q_0 = -0.52^{+0.15}_{-0.12}$, $j_0 = 1.5^{+1.1}_{-1.2}$, and $H_0 = 71.84^{+0.25}_{-0.19}$ km/s/Mpc. The paper concludes that the (2,1) Padé approximation explains Pantheon+ data at low and high redshifts and agrees with earlier theoretical arguments favoring numerator order one higher than denominator order.
Load-bearing premise
The ranking that selects (2,1) depends on AIC/BIC differences that the paper itself calls not significant, and the likelihood function and the effective number of free parameters used in those criteria are never specified.
Editorial extensions
If this is right
- If the conclusion is correct, the (2,1) Padé form should be adopted as the default rational cosmographic reconstruction for Pantheon+ and similar supernova distance data, replacing ad hoc order choices.
- The cross-validation procedure gives a template for order selection on other cosmological observables, such as Hubble parameter or BAO data, without assuming a specific cosmological model.
- The fitted parameter values from the (2,1) fit provide a model-independent reference point against which parameter estimates from full cosmological model fits can be checked.
- The result reinforces the rule that numerator order one above denominator order is preferred, because that pattern now emerges from data alone rather than from theoretical convenience.
Reading between the lines
- A natural next test is whether (2,1) still wins under repeated 10-fold cross-validation with many random seeds; the paper reports no error bars on the AIC/BIC differences, so the stability of the ranking is an open question.
- Applying the same data-driven scheme to independent distance catalogs would reveal whether (2,1) is a feature of the Pantheon+ sample specifically or of the underlying distance-redshift relation more generally.
- If the AIC/BIC gaps are within noise, the pipeline could be extended to score candidate orders by predictive calibration, such as the coverage of validation residuals, rather than by point estimates of information criteria.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes a data-driven procedure, based on k-fold cross-validation, to select the order of a Padé polynomial used to reconstruct the luminosity distance from type-Ia supernova data. The procedure is applied to the Pantheon+ sample, and on the basis of average MSE, AIC, and BIC scores in 10-fold CV the authors conclude that the (2,1) Padé approximation is optimal and that the scheme has a 'remarkable ability to distinguish' Padé orders. The paper also reports MCMC constraints on the (2,1) model parameters.
Significance. If the statistical procedure were fully specified and validated, the paper would offer a practical, data-driven alternative to the heuristic rules previously used to choose Padé orders in cosmography, and its conclusion that (2,1) is optimal agrees with earlier work by Capozziello et al. (2020). The manuscript is honest about limitations such as dependence on the Pantheon+ dataset and on the data-splitting ratio, and it provides explicit Padé expressions. However, the paper's own evidence does not yet support the strength of the claimed conclusion: the likelihood underlying AIC/BIC is undefined, the CV scores have no uncertainties, and several numerical and consistency issues remain.
major comments (6)
- [Section 3.2, Eqs. (11)-(12)] The AIC and BIC values cannot be reproduced because the maximum likelihood L_max is never defined. It is not stated whether L_max is constructed from the χ2 of Eq. (1) with the full Pantheon+ covariance matrix or from a Gaussian likelihood based on the MSE of Eq. (10); the two choices lead to different AIC/BIC values. Furthermore, the number of free parameters m is ambiguous: Eq. (6) contains j0 as a coefficient, the 'where' block after Eq. (9) fixes j0 = 1, and Table 1 reports a fitted j0 = 1.5^{+1.1}_{-1.2}. Since AIC and BIC depend linearly on m, this ambiguity can change the penalty and could flip the ranking of Padé orders. The authors should state L_max explicitly and give m for each candidate order under a consistent treatment of j0.
- [Section 4] The conclusion that the (2,1) Padé approximation is optimal is not statistically supported by the reported scores. The authors state that 'the difference of average of BIC and AIC is not significant,' yet they select (2,1) by looking at insets that amplify those small differences. No uncertainty estimates (e.g., standard errors over the ten folds or over repeated random splits) are reported for the average MSE, AIC, or BIC, so the ranking may be an artifact of the random fold partition. Please report per-fold values, standard errors, and a paired significance test for the AIC/BIC differences, and specify the number of repeats and random seed used in Steps 1-3.
- [Sections 2 and 4, Step 5] The sample size used in the analysis is inconsistent. The data section states that Pantheon+ comprises '1701 light curves of 1550 unique' supernovae, while Eq. (1) and Step 5 refer to '1071 SNe Ia data' from Koussour et al. (2024). Since n enters the BIC in Eq. (12) and determines the construction of the CV folds, the paper must clarify which of these samples is used and how the number 1071 is obtained.
- [Equations (6)-(9) and Table 1] The Padé expressions for the luminosity distance are written as 1/H0 times a dimensionless combination of z and the cosmographic coefficients, with no factor of c. In standard units, the leading factor for d_L should be c/H0, or an explicit convention c=1 must be stated. As written, substituting H0 in km/s/Mpc gives a quantity whose units are not Mpc, so Eq. (2) would not produce the correct distance modulus. The authors should introduce c explicitly or declare the unit convention.
- [Section 4, Step 2] The pre-selection of candidate Padé polynomials using the threshold 'average of MSE less than 0.05' is introduced without any statistical justification. This ad hoc cutoff can remove a model with slightly higher MSE but substantially better AIC/BIC, and it is applied to an average that carries no reported uncertainty. Please justify the threshold or omit the pre-selection step and compare all orders.
- [Figures 2-8] The manuscript as provided does not contain the actual figures that hold the numerical evidence: Figures 2-8 appear as placeholders such as 'Figure ??' in the text, and Figures 1 and 2 share the same caption. Without these figures and the underlying numerical values, the reported CV scores and MCMC contours cannot be inspected. The figures and the exact numbers must be supplied for the central claim to be verifiable.
minor comments (5)
- [References] The reference to Akaike et al. (1973) in the reference list is incomplete; it should include the full title and publisher information.
- [Figures 1 and 2] Figure 1 and Figure 2 have identical captions; please differentiate them to indicate which shows the preliminary candidates and which shows the final fits.
- [Table 1] The MCMC setup (priors, number of walkers, chain length, burn-in) is not described, so the reported parameter uncertainties in Table 1 cannot be reproduced.
- [Throughout] The text has several typographical and grammatical errors, e.g., 'red-shifts' in the abstract and 'Sec.4 present a comparison' in Section 1; a careful language edit is needed.
- [Figure 8] The notation '1 δ, and 2 δ' in Figure 8 should be replaced by the standard '68% and 95% confidence regions'.
Circularity Check
No load-bearing circularity: the 10-fold CV order selection is data-driven and benchmarked externally; the only self-citation is non-load-bearing, and the ΛCDM-constrained Padé templates and unquantified AIC/BIC are correctness concerns, not circular reductions.
full rationale
The paper's chain is: adopt Padé templates P2,1, P2,2, P3,1, P3,2 from Capozziello et al. (2020) (Sec. 3.1, Eqs. 6-9); split Pantheon+ data into k folds; compute average validation MSE, AIC, and BIC per order; rank them; pick order (2,1); then check consistency with prior external work (Capozziello et al. 2020; Aviles et al. 2014; Petreca et al. 2024). No step in this chain makes the output equivalent to the input by construction. The order ranking is an empirical outcome: nothing in the ΛCDM coefficient hierarchy in the 'where' block forces the (2,1) rational form to win over (2,2), (3,1), or (3,2). The AIC/BIC formulas (Eqs. 11-12) are never instantiated with a stated Lmax or per-model parameter count m, so the reported scores cannot be reproduced, and the paper itself admits 'the difference of average of BIC and AIC is not significant' (Sec. 4) before selecting (2,1) from amplified insets; those are statistical-validity and reproducibility flaws, not circularity. The templates are fiducial-model-dependent (q0 = 3/2 Ωm0 − 1, j0 = 1, s0 = 1 − 9/2 Ωm0, ...), which undercuts the 'non-parametric / only based on observations' framing, but this is a model-dependence caveat: the same fiducial expansion is used for all four candidates and does not predetermine the winner. The only self-citations (Yu et al. 2020; Yu et al. 2021 in Sec. 1) are introductory remarks and are not load-bearing; methodological support comes from external citations (Allen 1974; Kohavi 1995; Zhang & Yang 2015). Hence no circular step is exhibited.
Assumptions & free parameters
free parameters (4)
- H0 =
71.84 +0.25/-0.19 km/s/Mpc (68.3% C.L., Table 1)
- q0 (or Ωm0) =
-0.52 +0.15/-0.12 (Table 1)
- j0 =
1.5 +1.1/-1.2 (Table 1)
- s0, l0, m0 =
not reported
assumptions (4)
- standard math Padé approximants are constructed from the Taylor expansion of the luminosity distance using the standard coefficient-matching conditions (Eqs. 3-5).
- domain assumption The cosmographic parameters are linked to a flat ΛCDM model through q0 = (3/2)Ωm0 - 1, j0 = 1, s0 = 1 - (9/2)Ωm0, etc. (Sec. 3.1).
- ad hoc to paper Candidate Padé orders are pre-selected using an ad hoc threshold that the average MSE must be less than 0.05 (Sec. 4, Step 2).
- ad hoc to paper AIC and BIC are computed as fold-averaged values over cross-validation folds (Sec. 3.3).
Cite this review
Pith. "Pith review of The optimal Pad\'{e} polynomial for reconstruction of luminosity distance based on 10-fold cross-validation." pith.science (2026). https://pith.science/paper/VULVKM6P
@misc{pith2026250708695,
author = {Pith},
title = {Pith review of: The optimal Pad\'e polynomial for reconstruction of luminosity distance based on 10-fold cross-validation},
year = {2026},
howpublished = {\url{https://pith.science/paper/VULVKM6P}},
note = {Machine review of arXiv:2507.08695}
}
read the original abstract
The cosmography known as the Pad\'{e} polynomials has been widely used in the reconstruction of luminosity distance, and the orders of Pad\'{e} polynomials influence the reconstructed result derived from Pad\'{e} approximation. In this paper, we present a more general scheme of selecting optimal Pad\'{e} polynomial for reconstruction of luminosity distance based on 10-fold cross-validation. Then the proposed scheme is applied to Pantheon+ dataset. The numerical results clearly indicate that the proposed procedure has a remarkable ability to distinguish Pad\'{e} approximations with different orders for the reconstruction of the luminosity distance. We conclude that the (2,1) Pad\'e approximation is the optimal approach that can well explain Pantheon+ data at low and high red-shifts. Future applications of this scheme could help choose the optimal model that is more suitable for cosmological observation data at hand and gain a deeper understanding of the universe.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
, " * write output.state after.block = add.period write newline
ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...
-
[2]
write newline
" write newline "" before.all 'output.state := FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix "arXiv" = new.block " " eprint * " " * new.block " " eprint * " " * if if if FUNCTION format.doi doi empty "" " " doi * " " * if FUNCTION format.pid doi empty eprint empty ur...
-
[3]
thebibliography [1] 20pt to REFERENCES 6pt =0pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key on reference command E...
arXiv 2021
-
[4]
Akaike, H., Petrov, B. N., & Czaki, F. 1973, 2nd Int.Sympo.on Information Theory
work page 1973
-
[5]
Allen, D. M. 1974, Technometrics, 16, 125
work page 1974
-
[6]
2010, Statistics Surveys, 4, 40 , 10.1214/09-SS054
Arlot, S., & Celisse, A. 2010, Statistics Surveys, 4, 40 , 10.1214/09-SS054
doi:10.1214/09-ss054 2010
-
[7]
Aviles, A., Bravetti, A., Capozziello, S., & Luongo, O. 2014, Phys. Rev. D, 90, 043531, 10.1103/PhysRevD.90.043531
-
[8]
Bassett, B. A., & Hlozek, R. 2009, Dark Energy Observational & Theoretical Approaches, 43, 246
work page 2009
Show all 30 references
-
[9]
2020, Monthly Notices of the Royal Astronomical Society, 494, 2576, 10.1093/mnras/staa871
Capozziello, S., D’Agostino, R., & Luongo, O. 2020, Monthly Notices of the Royal Astronomical Society, 494, 2576, 10.1093/mnras/staa871
2020 doi
-
[10]
2019, , 484, 4484, 10.1093/mnras/stz176
Capozziello , S., & Ruchika , Anjan A., S. 2019, , 484, 4484, 10.1093/mnras/stz176
2019 doi
-
[11]
Fisher, R. A. 1922, Philosophical Transactions of the Royal Society of London, 222, 10.1007/978-1-4612-0919-5_2
1922 doi
-
[12]
W., Lang, D., & Goodman, J
Foreman-Mackey, D., Hogg, D. W., Lang, D., & Goodman, J. 2013, Publications of the Astronomical Society of the Pacific, 125, 306, 10.1086/670067
2013 doi
-
[13]
2019, Energies, 12, 1, 10.3390/en12050915
Herisanu, N., Marinca, V., Madescu, G., & Dragan, F. 2019, Energies, 12, 1, 10.3390/en12050915
2019 doi
-
[14]
F., Gomes, M
Jesus, J. F., Gomes, M. J., Holanda, R. F., & Nunes, R. C. 2025, Journal of Cosmology and Astroparticle Physics, 2025, 088, 10.1088/1475-7516/2025/01/088
2025 doi
-
[15]
1995, in International joint conference on Artificial intelligence
Kohavi, R. 1995, in International joint conference on Artificial intelligence
1995
-
[16]
Koussour, M., Myrzakulov, N., & Ali, M. K. M. 2024, Journal of High Energy Astrophysics, 42, 96
2024
-
[17]
2011, Communications in Theoretical Physics, 56, 525, 10.1088/0253-6102/56/3/24
Li, M., Li, X.-D., Wang, S., & Wang, Y. 2011, Communications in Theoretical Physics, 56, 525, 10.1088/0253-6102/56/3/24
2011 doi
-
[18]
2016, Monthly Notices of the Royal Astronomical Society, 460, 273, 10.1093/mnras/stw964
Mukherjee , A. 2016, Monthly Notices of the Royal Astronomical Society, 460, 273, 10.1093/mnras/stw964
2016 doi
-
[19]
1999, The Astrophysical Journal, 517, 565, 10.1086/307221
Perlmutter, S., Aldering, G., Goldhaber, G., et al. 1999, The Astrophysical Journal, 517, 565, 10.1086/307221
1999 doi
-
[20]
T., Benetti, M., & Capozziello, S
Petreca, A. T., Benetti, M., & Capozziello, S. 2024, Physics of the Dark Universe, 44, 101453, https://doi.org/10.1016/j.dark.2024.101453
2024
- [21]
-
[22]
1978, The Annals of Statistics, 6, 461
Schwarz, G. 1978, The Annals of Statistics, 6, 461
1978
-
[23]
M., et al
Scolnic, D. M., et al. 2018, The Astrophysical Journal, 859, 101, 10.3847/1538-4357/aab9bb
2018 doi
-
[24]
2022, The Astrophysical Journal, 938, 113, 10.3847/1538-4357/ac8b7a
---. 2022, The Astrophysical Journal, 938, 113, 10.3847/1538-4357/ac8b7a
2022 doi
-
[25]
2014, Journal of Cosmology and Astroparticle Physics, 2014, 045, 10.1088/1475-7516/2014/01/045
Wei, H., Yan, X.-P., & Zhou, Y.-N. 2014, Journal of Cosmology and Astroparticle Physics, 2014, 045, 10.1088/1475-7516/2014/01/045
2014 doi
-
[26]
1979, Pade Approximation and Its Applications (Springer-Verlag,)
Wuytack, L. 1979, Pade Approximation and Its Applications (Springer-Verlag,)
1979
-
[27]
2020, Physics of the Dark Universe, 30, 100734, https://doi.org/10.1016/j.dark.2020.100734
Yu, B., Wang, Z.-H., Liu, D.-Z., & Zhang, T.-J. 2020, Physics of the Dark Universe, 30, 100734, https://doi.org/10.1016/j.dark.2020.100734
2020
-
[28]
2021, Physics of the Dark Universe, 31, 100772, https://doi.org/10.1016/j.dark.2021.100772
Yu, B., Zhang, J.-C., Zhang, T.-J., & Zhang, T. 2021, Physics of the Dark Universe, 31, 100772, https://doi.org/10.1016/j.dark.2021.100772
2021
-
[29]
2015, Journal of Econometrics, 187, 95, https://doi.org/10.1016/j.jeconom.2015.02.006
Zhang, Y., & Yang, Y. 2015, Journal of Econometrics, 187, 95, https://doi.org/10.1016/j.jeconom.2015.02.006
2015 doi
-
[30]
2016, European Physical Journal C, 76, 281, 10.1140/epjc/s10052-016-4091-z
Zhou , Y.-N., Liu , D.-Z., Zou , X.-B., & Wei , H. 2016, European Physical Journal C, 76, 281, 10.1140/epjc/s10052-016-4091-z
2016 doi
Reviewed August 6, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.