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A090593 as a simple table

na(n)
12
23
37
447
531
6191
7127
81021
93583
104093
116143
1216381
138191
1481919
15262139
161114111
17131071
18786431
19524287
204194301
2114680063
2216777213
2367108859
24654311423
25671088637
26738197503
27536870909
285368709117
293758096383
3034359738337
312147483647
3221474836479

[2,3,7,47,31,191,127,1021,3583,4093,6143,16381,
 8191,81919,262139,1114111,131071,786431,524287,
 4194301,14680063,16777213,67108859,654311423,
 671088637,738197503,536870909,5368709117,
 3758096383,34359738337,2147483647,21474836479]