Computing Minimum Tile Sets to Self-Assemble Colors Patterns
  Patterned self-assembly tile set synthesis (PATS) aims at finding a minimum tile set to uniquely self-assemble a given rectangular color pattern. For $k \ge 1$, $k$-PATS is a variant of PATS that restricts input patterns to those with at most $k$ colors. We prove the {\bf NP}-hardness of 29-PATS, where the best known is that of 60-PATS. 
Aleck C. Johnsen, Ming-Yang Kao, Shinnosuke Seki
cs.CC