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A233930 as a simple table

na(n)
13
27
317
431
5127
6257
73583
85119
96143
108191
1165537
12131071
13524287
147340033
1514680063
16104857601
17109051903
18167772161
19469762049
202013265921
212147483647
2221474836479
2351539607551
24206158430209
25824633720831
262748779069441
276597069766657
2826388279066623

[3,7,17,31,127,257,3583,5119,6143,8191,65537,
 131071,524287,7340033,14680063,104857601,
 109051903,167772161,469762049,2013265921,
 2147483647,21474836479,51539607551,206158430209,
 824633720831,2748779069441,6597069766657,
 26388279066623]