Efficient List-Decoding with Constant Alphabet and List Sizes

We present an explicit and efficient algebraic construction of capacity-achieving list decodable codes with both constant alphabet and constant list sizes. More specifically, for any $R \in (0,1)$ and $ε>0$, we give an algebraic construction of an infinite family of error-correcting codes of rate $R$, over an alphabet of size $(1/ε)^{O(1/ε^2)}$, that can be list decoded from a $(1-R-ε)$-fraction of errors with list size at most $\exp(\mathrm{poly}(1/ε))$. Moreover, the codes can be encoded in time $\mathrm{poly}(1/ε, n)$, the output list is contained in a linear subspace of dimension at most $\mathrm{poly}(1/ε)$, and a basis for this subspace can be found in time $\mathrm{poly}(1/ε, n)$. Thus, both encoding and list decoding can be performed in fully polynomial-time $\mathrm{poly}(1/ε, n)$, except for pruning the subspace and outputting the final list which takes time $\exp(\mathrm{poly}(1/ε))\cdot\mathrm{poly}(n)$. Our codes are quite natural and structured. Specifically, we use algebraic-geometric (AG) codes with evaluation points restricted to a subfield, and with the message space restricted to a (carefully chosen) linear subspace. Our main observation is that the output list of AG codes with subfield evaluation points is contained in an affine shift of the image of a block-triangular-Toeplitz (BTT) matrix, and that the list size can potentially be reduced to a constant by restricting the message space to a BTT evasive subspace, which is a large subspace that intersects the image of any BTT matrix in a constant number of points. We further show how to explicitly construct such BTT evasive subspaces, based on the explicit subspace designs of Guruswami and Kopparty (Combinatorica, 2016), and composition.