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A003260 as a simple table

na(n)
13
27
331
4127
589
68191
7131071
8524287
9178481
102089
112147483647
12616318177
13164511353
142099863
1513264529
1620394401
173203431780337
182305843009213693951

[3,7,31,127,89,8191,131071,524287,178481,2089,
 2147483647,616318177,164511353,2099863,13264529,
 20394401,3203431780337,2305843009213693951]