a(n) = 2^n concatenated with itself.

Terms

a(0) =11a(1) =22a(2) =44a(3) =88a(4) =1616a(5) =3232a(6) =6464a(7) =128128a(8) =256256a(9) =512512a(10) =10241024a(11) =20482048a(12) =40964096a(13) =81928192a(14) =1638416384a(15) =3276832768a(16) =6553665536a(17) =131072131072a(18) =262144262144a(19) =524288524288