A109827 - OEIS

A109827 as a simple table

n	a(n)
0	0
1	1
2	10
3	11
4	20
5	21
6	100
7	101
8	110
9	111
10	120
11	121
12	1000
13	1001
14	1010
15	1011
16	1020
17	1021
18	1100
19	1101
20	1110
21	1111
22	1120
23	1121
24	2000
25	2001
26	2010
27	2011
28	2020
29	2021
30	2100
31	2101
32	2110
33	2111
34	2120
35	2121
36	10000
37	10001
38	10010
39	10011
40	10020
41	10021
42	10100
43	10101

[0,1,10,11,20,21,100,101,110,111,120,121,1000, 1001,1010,1011,1020,1021,1100,1101,1110,1111,1120, 1121,2000,2001,2010,2011,2020,2021,2100,2101,2110, 2111,2120,2121,10000,10001,10010,10011,10020, 10021,10100,10101]