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A003585
as a simple table
n
a(n)
1
3
2
7
3
21
4
31
5
33
6
43
7
67
8
79
9
87
10
133
11
217
12
219
13
223
14
253
15
307
16
391
17
679
18
2719
[3,7,21,31,33,43,67,79,87,133,217,219,223,253,307, 391,679,2719]