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A117387 as a simple table

na(n)
02
12
23
37
417
531
661
7127
8257
9509
101021
112053
124093
138191
1416381
1532771
1665537
17131071
18262147
19524287
201048573
212097143
224194301
238388617
2416777213
2533554467
2667108859
27134217757
28268435459
29536870909
301073741827
312147483647
324294967291
338589934583

[2,2,3,7,17,31,61,127,257,509,1021,2053,4093,8191,
 16381,32771,65537,131071,262147,524287,1048573,
 2097143,4194301,8388617,16777213,33554467,
 67108859,134217757,268435459,536870909,1073741827,
 2147483647,4294967291,8589934583]