a(n) = (4*16^(n-1)-1)/3.

A144864

a(n) = (4*16^(n-1)-1)/3.

Terms

    a(0) =1a(1) =21a(2) =341a(3) =5461a(4) =87381a(5) =1398101a(6) =22369621a(7) =357913941a(8) =5726623061a(9) =91625968981

External references