a(n) is the number of n-digit positive integers with exactly two distinct base 10 digits.

Terms

a(0) =0a(1) =81a(2) =243a(3) =567a(4) =1215a(5) =2511a(6) =5103a(7) =10287a(8) =20655a(9) =41391a(10) =82863a(11) =165807a(12) =331695a(13) =663471a(14) =1327023a(15) =2654127a(16) =5308335a(17) =10616751a(18) =21233583a(19) =42467247a(20) =84934575a(21) =169869231a(22) =339738543a(23) =679477167a(24) =1358954415a(25) =2717908911a(26) =5435817903a(27) =10871635887a(28) =21743271855a(29) =43486543791