Numbers n such that string 2,1 occurs in the base 10 representation of n but not of n-1.

Terms

a(0) =21a(1) =121a(2) =210a(3) =221a(4) =321a(5) =421a(6) =521a(7) =621a(8) =721a(9) =821a(10) =921a(11) =1021a(12) =1121a(13) =1210a(14) =1221a(15) =1321a(16) =1421a(17) =1521a(18) =1621a(19) =1721a(20) =1821a(21) =1921a(22) =2021a(23) =2100a(24) =2210a(25) =2221a(26) =2321a(27) =2421a(28) =2521a(29) =2621