Improved Field Size Bounds for Higher Order MDS Codes

Higher order MDS codes are an interesting generalization of MDS codes recently introduced by Brakensiek et al., (2023). In later works, they were shown to be intimately connected to optimally list-decodable codes and maximally recoverable tensor codes. Therefore (explicit) constructions of higher order MDS codes over small fields is an important open problem. Higher order MDS codes are denoted by $\rm {MDS}(\ell)$ where $\ell $ denotes the order of generality, $\rm {MDS}(2)$ codes are equivalent to the usual MDS codes. The best prior lower bound on the field size of an ${[}n,k{]}$ - $\rm {MDS}(\ell)$ codes is $\Omega _{\ell } (n^{\ell -1})$ , whereas the best known (non-explicit) upper bound is $O_{\ell } (n^{k(\ell -1)})$ which is exponential in the dimension. In this work, we nearly close this exponential gap between upper and lower bounds. We show that an ${[}n,k{]}$ - $\rm {MDS}(3)$ codes requires a field of size $\Omega _{k}(n^{k-1})$ , which is close to the known upper bound. Using the connection between higher order MDS codes and optimally list-decodable codes, we show that even for a list size of 2, a code which meets the optimal list-decoding Singleton bound requires exponential field size; this resolves an open question by Shangguan and Tamo, (2020). We also give explicit constructions of ${[}n,k{]}$ - $\rm {MDS}(\ell)$ code over fields of size $n^{(\ell k)^{O(\ell k)}}$ . The smallest non-trivial case where we still do not have optimal constructions is ${[}n,3{]}$ - $\rm {MDS}(3)$ . In this case, the known lower bound on the field size is $\Omega (n^{2})$ and the best known upper bounds are $O(n^{5})$ for a non-explicit construction and $O(n^{32})$ for an explicit construction. In this paper, we give an explicit construction over fields of size $O(n^{3})$ which comes very close to being optimal.