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A157240 as a simple table

na(n)
02
1-1
2-7
3-7
417
565
665
7-127
8-511
9-511
101025
114097
124097
13-8191
14-32767
15-32767
1665537
17262145
18262145
19-524287
20-2097151
21-2097151
224194305
2316777217
2416777217
25-33554431
26-134217727
27-134217727
28268435457
291073741825

[2,-1,-7,-7,17,65,65,-127,-511,-511,1025,4097,
 4097,-8191,-32767,-32767,65537,262145,262145,
 -524287,-2097151,-2097151,4194305,16777217,
 16777217,-33554431,-134217727,-134217727,
 268435457,1073741825]