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A028399 as a simple table

na(n)
20
34
412
528
660
7124
8252
9508
101020
112044
124092
138188
1416380
1532764
1665532
17131068
18262140
19524284
201048572
212097148
224194300
238388604
2416777212
2533554428
2667108860
27134217724
28268435452
29536870908
301073741820
312147483644
324294967292
338589934588
3417179869180

[0,4,12,28,60,124,252,508,1020,2044,4092,8188,
 16380,32764,65532,131068,262140,524284,1048572,
 2097148,4194300,8388604,16777212,33554428,
 67108860,134217724,268435452,536870908,1073741820,
 2147483644,4294967292,8589934588,17179869180]