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A247823
as a simple table
n
a(n)
1
7
2
8191
3
131071
4
524287
5
2147483647
6
2305843009213693951
7
618970019642690137449562111
[7,8191,131071,524287,2147483647, 2305843009213693951,618970019642690137449562111]