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

Terms

a(0) =12a(1) =112a(2) =120a(3) =212a(4) =312a(5) =412a(6) =512a(7) =612a(8) =712a(9) =812a(10) =912a(11) =1012a(12) =1112a(13) =1120a(14) =1200a(15) =1312a(16) =1412a(17) =1512a(18) =1612a(19) =1712a(20) =1812a(21) =1912a(22) =2012a(23) =2112a(24) =2120a(25) =2212a(26) =2312a(27) =2412a(28) =2512a(29) =2612