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A307503 as a simple table

na(n)
02
12
23
37
431
531
6127
7127
81021
93583
104093
116143
128191
138191
1481919
15131071
16131071
17131071
18524287
19524287
204194301
2114680063
2216777213
2367108859
24536870909
25536870909
26536870909
27536870909
282147483647
292147483647
302147483647
312147483647
3221474836479

[2,2,3,7,31,31,127,127,1021,3583,4093,6143,8191,
 8191,81919,131071,131071,131071,524287,524287,
 4194301,14680063,16777213,67108859,536870909,
 536870909,536870909,536870909,2147483647,
 2147483647,2147483647,2147483647,21474836479]