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A243305 as a simple table

na(n)
13
23
35
45
517
65
765
817
965
1017
111025
1217
134097
1465
15257
16257
1765537
1865
19262145
20257
214097
221025
234194305
24257
251048577
264097
27262145
284097
29268435457
30257
311073741825
3265537
331048577
3465537
3516777217
364097
3768719476737
38262145
3916777217
4065537
411099511627777

[3,3,5,5,17,5,65,17,65,17,1025,17,4097,65,257,257,
 65537,65,262145,257,4097,1025,4194305,257,1048577,
 4097,262145,4097,268435457,257,1073741825,65537,
 1048577,65537,16777217,4097,68719476737,262145,
 16777217,65537,1099511627777]