A Multi-Order Extension of Fractional HBVMs (FHBVMs)
Pith reviewed 2026-05-17 01:31 UTC · model grok-4.3
The pith
Fractional HBVMs extend to solve fractional differential equations with multiple different derivative orders.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Fractional HBVMs have so far been defined for systems with the same fractional derivative order. This paper proposes an extension of FHBVMs for addressing fractional multi-order problems, providing full details for such an approach. A corresponding Matlab code, handling the case of two different fractional orders, is also made available, proving very effective for numerically solving these problems.
What carries the argument
The direct adaptation of the single-order FHBVM collocation structure to accommodate distinct fractional orders within one system of equations.
If this is right
- The extended methods produce numerical solutions for fractional systems containing two or more distinct derivative orders.
- The supplied Matlab code implements the case of exactly two orders and can be run directly on such problems.
- The approach remains competitive with other existing numerical schemes for multi-order fractional equations.
- The detailed construction allows straightforward generalization to problems with more than two orders.
Where Pith is reading between the lines
- The same adaptation pattern might apply to variable-order fractional derivatives without major redesign.
- Applications in viscoelasticity or anomalous diffusion that naturally produce multi-order models could use this solver as a starting point.
- Stability proofs for arbitrary numbers of orders would strengthen the method for general use.
Load-bearing premise
The single-order FHBVM framework can be directly adapted to multiple orders while preserving stability, accuracy, and computational efficiency without requiring substantial new theoretical analysis.
What would settle it
A concrete multi-order test problem on which the extended method loses its expected convergence rate or exhibits instability would show that the direct adaptation fails to carry over the desired properties.
Figures
read the original abstract
The efficient numerical solution of fractional differential equations has been recently tackled through the definition of Fractional HBVMs (FHBVMs), a class of Runge-Kutta type methods. Corresponding Matlab (c) codes have been also made available on the internet, proving to be very competitive w.r.t. existing ones. However, so far, FHBVMs have been given for solving systems of fractional differential equations with the same order of fractional derivative, whereas the numerical solution of multi-order problems (i.e., problems in which different orders of fractional derivatives occur) has not been handled, yet. Due to their relevance in applications, in this paper we propose an extension of FHBVMs for addressing fractional multi-order problems, providing full details for such an approach. A corresponding Matlab (c) code, handling the case of two different fractional orders, is also made available, proving very effective for numerically solving these problems.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper extends Fractional HBVMs (FHBVMs) from single-order to multi-order fractional differential equations. It adapts the quadrature and collocation framework by applying separate discrete fractional integral operators for each order, supplies implementation details, and provides a Matlab code for the two-order case, claiming the resulting scheme remains competitive for solving such problems.
Significance. If the extension preserves the accuracy, stability, and efficiency of the original FHBVMs for coupled multi-order systems, it would fill a practical gap, since multi-order fractional models appear in applications. The public code is a positive feature for reproducibility.
major comments (2)
- The central construction in the multi-order extension section adapts the single-order FHBVM quadrature and collocation conditions by applying separate discrete fractional integral operators for each order α and β. However, no derivation or verification is given of the modified order conditions that arise from the interaction of the two distinct fractional kernels when the orders are incommensurate and the equations are coupled; this is load-bearing for the claim that the scheme retains the original accuracy and stability properties.
- Numerical experiments section: the reported tests use commensurate or simple cases and do not include a direct check (e.g., via order tables or stability plots) that the multi-order scheme satisfies the same convergence rates and stability bounds as the single-order FHBVM when the fractional orders differ and the system is coupled.
minor comments (2)
- Notation for the multi-order discrete integral operators should be introduced with an explicit comparison table to the single-order operators to prevent reader confusion.
- The abstract and introduction could add a one-sentence statement of the precise order conditions that are being preserved (or modified) under the extension.
Simulated Author's Rebuttal
We thank the referee for the careful reading of the manuscript and the constructive comments. We address the major comments point by point below, indicating where revisions will be made to strengthen the presentation.
read point-by-point responses
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Referee: The central construction in the multi-order extension section adapts the single-order FHBVM quadrature and collocation conditions by applying separate discrete fractional integral operators for each order α and β. However, no derivation or verification is given of the modified order conditions that arise from the interaction of the two distinct fractional kernels when the orders are incommensurate and the equations are coupled; this is load-bearing for the claim that the scheme retains the original accuracy and stability properties.
Authors: We agree that an explicit derivation of the order conditions for the multi-order case would clarify the extension. The construction applies independent discrete fractional integral operators to each term according to its order, so that the local truncation error analysis proceeds componentwise even when the orders are incommensurate and the right-hand side is coupled. We will add a short subsection deriving the resulting order conditions and confirming that the accuracy and stability properties carry over from the single-order setting. revision: yes
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Referee: Numerical experiments section: the reported tests use commensurate or simple cases and do not include a direct check (e.g., via order tables or stability plots) that the multi-order scheme satisfies the same convergence rates and stability bounds as the single-order FHBVM when the fractional orders differ and the system is coupled.
Authors: The existing experiments illustrate the method on multi-order problems, some of which already involve distinct orders. To provide the requested direct verification, we will augment the numerical section with additional tests on coupled systems with incommensurate orders, including convergence tables for successive step-size reductions and a brief discussion of observed stability behavior. revision: yes
Circularity Check
No significant circularity; extension provides independent implementation details
full rationale
The paper explicitly states it provides full details for adapting FHBVMs to multi-order fractional problems and releases corresponding Matlab code for two orders. This constitutes new implementation steps rather than a reduction of the central claim to prior inputs by construction, fitted parameters, or unverified self-citation chains. The derivation chain is self-contained against the stated goal of handling incommensurate orders via separate operators, with no quoted equations showing the multi-order scheme equaling its single-order inputs without additional content. This is the normal honest outcome for an extension paper that supplies concrete new material.
Axiom & Free-Parameter Ledger
Reference graph
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