Here the first 13 I think. But I dont see the pattern. What is it?

1 : '' # empty string.
2 : 0
3 : 00
4 : 11
5 : 000
6 : 011
7 : 110
8 : 0000
9 : 0011
10: 0110
11: 1001
12: 1100
13: 1111
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--- demerphq
my friends call me, usually because I'm late....