Number of length-2n binary strings of the form xxyy, with x and y both nonempty.

Terms

a(0) =0a(1) =4a(2) =14a(3) =42a(4) =116a(5) =292a(6) =722a(7) =1682a(8) =3920a(9) =8820a(10) =19770a(11) =43578a(12) =95540a(13) =207244a(14) =447812a(15) =960676a(16) =2053292a(17) =4368094a(18) =9262226a(19) =19571396a(20) =41243844a(21) =86678272a(22) =181752522a(23) =380276248a(24) =794120918