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A290620 as a simple table

na(n)
01
12
27
315
428
563
6126
7255
8511
91023
102032
114095
128160
1316383
1432752
1565535
16130840
17262143
18524172
191048575
202097118
214194303
228388607
2316777215
2433554366
2567108863
26134216188
27268435455
28536867776
291073741823
302147483136
314294967295
328589925888

[1,2,7,15,28,63,126,255,511,1023,2032,4095,8160,
 16383,32752,65535,130840,262143,524172,1048575,
 2097118,4194303,8388607,16777215,33554366,
 67108863,134216188,268435455,536867776,1073741823,
 2147483136,4294967295,8589925888]