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A059990 as a simple table

na(n)
11
21
37
45
531
67
7127
885
9511
10341
112047
12455
138191
145461
1532767
1621845
17131071
189709
19524287
20349525
212097151
221398101
238388607
241864135
2533554431
2622369621
27134217727
2889478485
29536870911
30119304647

[1,1,7,5,31,7,127,85,511,341,2047,455,8191,5461,
 32767,21845,131071,9709,524287,349525,2097151,
 1398101,8388607,1864135,33554431,22369621,
 134217727,89478485,536870911,119304647]