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A006515 as a simple table

na(n)
13
27
331
4127
52047
68191
7131071
8524287
98388607
102147483647
11137438953471
122199023255551
13576460752303423487
142305843009213693951
15147573952589676412927
169671406556917033397649407
17618970019642690137449562111
18158456325028528675187087900671

[3,7,31,127,2047,8191,131071,524287,8388607,
 2147483647,137438953471,2199023255551,
 576460752303423487,2305843009213693951,
 147573952589676412927,9671406556917033397649407,
 618970019642690137449562111,
 158456325028528675187087900671]