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A066781 as a simple table

na(n)
12
22
34
44
516
64
764
816
964
1016
111024
1216
134096
1464
15256
16256
1765536
1864
19262144
20256
214096
221024
234194304
24256
251048576
264096
27262144
284096
29268435456
30256
311073741824
3265536
331048576
3465536
3516777216
364096
3768719476736
38262144
3916777216
4065536

[2,2,4,4,16,4,64,16,64,16,1024,16,4096,64,256,256,
 65536,64,262144,256,4096,1024,4194304,256,1048576,
 4096,262144,4096,268435456,256,1073741824,65536,
 1048576,65536,16777216,4096,68719476736,262144,
 16777216,65536]