A007623 - OEIS

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A007623 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	200
13	201
14	210
15	211
16	220
17	221
18	300
19	301
20	310
21	311
22	320
23	321
24	1000
25	1001
26	1010
27	1011
28	1020
29	1021
30	1100
31	1101
32	1110
33	1111
34	1120
35	1121
36	1200
37	1201
38	1210
39	1211
40	1220
41	1221
42	1300
43	1301
44	1310
45	1311
46	1320
47	1321
48	2000
49	2001
50	2010

[0,1,10,11,20,21,100,101,110,111,120,121,200,201, 210,211,220,221,300,301,310,311,320,321,1000,1001, 1010,1011,1020,1021,1100,1101,1110,1111,1120,1121, 1200,1201,1210,1211,1220,1221,1300,1301,1310,1311, 1320,1321,2000,2001,2010]