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A121620 as a simple table

na(n)
13
27
331
4127
5313968931
68191
7131071
8524287
9777809294098524691
1068629840493971
112147483647
12114867606414015793728780533209145917205659365404867510184121
1344487435359130133495783012898708551
141136791005963704961126617632861

[3,7,31,127,313968931,8191,131071,524287,
 777809294098524691,68629840493971,2147483647,
 11486760641401579372878053320914591720565936540486\
 7510184121,44487435359130133495783012898708551,
 1136791005963704961126617632861]