  Patterned self-assembly tile set synthesis (PATS) aims at finding a minimum
tile set to uniquely self-assemble a given rectangular (color) pattern. For k
>= 1, k-PATS is a variant of PATS that restricts input patterns to those with
at most $k$ colors. A computer-assisted proof has been recently proposed for
2-PATS by Kari et al. [arXiv:1404.0967 (2014)]. In contrast, the best known
manually-checkable proof is for the NP-hardness of 29-PATS by Johnsen, Kao, and
Seki [ISAAC 2013, LNCS 8283, pp.~699-710]. We propose a manually-checkable
proof for the NP-hardness of 11-PATS.
