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A186307
as a simple table
n
a(n)
1
2
2
3
3
5
4
7
5
11
6
13
7
17
8
31
9
61
10
73
11
181
12
241
13
421
14
757
15
1009
16
1321
17
1801
18
2161
[2,3,5,7,11,13,17,31,61,73,181,241,421,757,1009, 1321,1801,2161]