Optimal one-symbol redundancy conjecture for Lee-metric function-correcting codes

Let t1t\geq 1 and q2t(t+1)+1q\geq 2t(t+1)+1. For a function ff, consider the encoding

Enc(u)=(u,(pf(u))modq),\operatorname{Enc}(\boldsymbol{u})=(\boldsymbol{u},(pf(\boldsymbol{u}))\bmod q),

where p[2t:q/2]p\in[2t:\lfloor q/2\rfloor]. The code is required to function-correct for the Lee weight function, the Lee weight distribution function, and the modular sum function. The optimal one-symbol redundancy conjecture. For all such tt and qq, there exists a p[2t:q/2]p\in[2t:\lfloor q/2\rfloor] for which this encoding yields an FCLC for each of the three functions and achieves optimal redundancy 11. When q=2t(t+1)+1q=2t(t+1)+1, one may take p=2t+1p=2t+1. If true, this formalizes the observation that sufficiently large alphabets relative to tt permit function-correcting linear codes with a single parity symbol and optimal redundancy. The supplied text gives no resolution.

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Primary source

Hareesh K., Rashid Ummer N. T. and B. Sundar Rajan, “Plotkin-like Bound and Explicit Function-Correcting Code Constructions for Lee Metric Channels”, arXiv:2508.01702 (2026).

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