A spectral fractional Hirota bilinear formulation]{A spectral fractional Hirota bilinear formulation for a time-fractional nonlinear Schr\"odinger flow: one-soliton solution and obstruction to finite two-soliton truncation
Objective & Hypothesis:We define a skew-adjoint spectral time-fractional operator $\mathcal{D}_t^\alpha$ ($0<\alpha\le1$) by the Fourier multiplier $\widehat{\mathcal{D}_t^\alpha\varphi}(\omega)=\mathrm{i}\,\mathrm{sgn}(\omega)|\omega|^\alpha\hat\varphi(\omega)$.
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Key Findings & Data:Using the induced fractional Hirota $D_t^\alpha$-operator we derive a bilinear system for a time-fractional focusing NLS flow, recovering the classical Hirota form when $\alpha=1$.
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Conclusion & Impact:Numerical residual plots illustrate the mismatch.
We define a skew-adjoint spectral time-fractional operator $\mathcal{D}_t^\alpha$ ($0<\alpha\le1$) by the Fourier multiplier $\widehat{\mathcal{D}_t^\alpha\varphi}(\omega)=\mathrm{i}\,\mathrm{sgn}(\omega)|\omega|^\alpha\hat\varphi(\omega)$. This provides a single-valued exponential Hirota calculus. Using the induced fractional Hirota $D_t^\alpha$-operator we derive a bilinear system for a time-fractional focusing NLS flow, recovering the classical Hirota form when $\alpha=1$. A stationary bright one-soliton $\tau$-function is obtained with dispersion relation $\omega=k^{2/\alpha}$. For $0<\alpha<1$ we prove that the classical finite two-soliton Hirota truncation is generically inconsistent: interaction-mode matching forces identities such as $(2\omega_2-\omega_1)^\alpha=2\omega_2^\alpha-\omega_1^\alpha$, contradicting strict concavity. Numerical residual plots illustrate the mismatch.
Ray, S. (2026). A spectral fractional Hirota bilinear formulation]{A spectral fractional Hirota bilinear formulation for a time-fractional nonlinear Schr\"odinger flow: one-soliton solution and obstruction to finite two-soliton truncation. Proceedings of International Mathematical Sciences, Volume: 8 Issue: 1. https://doi.org/10.47086/pims.1866476
R. Grande, Space–time fractional nonlinear Schr¨odinger equation, SIAM J. Math. Anal. 51 (2019), 4172–4212. DOI: 10.1137/19M1247140.
S. Ray, A spectral fractional Hirota bilinear operator: analysis and application to a timefractional KdV equation, arXiv:2601.17347 (2026). DOI: 10.48550/arXiv.2601.17347.
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