A004044: Size of minimal covering code in {Z_3}^n with covering radius 1. Best known solutions for n <= 6. By Dmitry Kamenetsky, 27/07/2020. a(1)=1 (1) a(2)=3 (01, 10, 21) a(3)=5 (001, 010, 100, 111, 222) a(4)=9 (0011, 0120, 0202, 1022, 1101, 1210, 2000, 2112, 2221) a(5)=27 (00002, 00110, 00221, 01020, 01101, 01212, 02011, 02122, 02200, 10012, 10121, 10200, 11001, 11110, 11222, 12020, 12102, 12211, 20021, 20100, 20212, 21010, 21122, 21201, 22002, 22111, 22220) a(6)<=73 (000112, 000201, 000202, 001022, 001120, 001201, 002010, 002020, 002201, 010002, 010210, 011011, 011100, 011120, 012111, 012112, 012222, 020021, 020121, 020221, 020222, 021100, 021212, 022000, 022010, 022102, 100011, 100220, 101022, 101102, 101210, 102002, 102121, 102122, 110020, 110101, 110212, 111010, 111012, 111221, 112110, 112200, 120110, 120200, 121001, 121002, 121122, 122022, 122102, 122211, 200000, 200100, 200221, 201000, 201022, 201111, 202100, 202212, 210122, 210210, 211111, 211202, 212001, 212011, 212021, 220012, 220201, 221020, 221111, 221220, 222102, 222120, 222220)