Chromatic number of stage-n Menger sponge.

Terms

a(0) =4a(1) =11a(2) =34a(3) =133a(4) =566a(5) =2488a(6) =11056a(7) =49323a(8) =220373a(9) =985176a(10) =4405203a(11) =19699535a(12) =88096982a(13) =393978082a(14) =1761917118a(15) =7879521402a(16) =35238270419a(17) =157590299379a(18) =704765178272