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A222598 as a simple table

na(n)
15
23
37
415
5159
627
7127
8255
9511
101023
111819
124095
134255
1416383
1532767
1665535
1777671
18262143
19459759
201048575
212097151
224194303
237456539
2416777215
2533554431
2667108863
27134217727
28268435455
29125687199
301073741823
312147483647
324294967295
338589934591

[5,3,7,15,159,27,127,255,511,1023,1819,4095,4255,
 16383,32767,65535,77671,262143,459759,1048575,
 2097151,4194303,7456539,16777215,33554431,
 67108863,134217727,268435455,125687199,1073741823,
 2147483647,4294967295,8589934591]