4n concatenated with itself.

Terms

a(0) =44a(1) =88a(2) =1212a(3) =1616a(4) =2020a(5) =2424a(6) =2828a(7) =3232a(8) =3636a(9) =4040a(10) =4444a(11) =4848a(12) =5252a(13) =5656a(14) =6060a(15) =6464a(16) =6868a(17) =7272a(18) =7676a(19) =8080a(20) =8484a(21) =8888a(22) =9292a(23) =9696a(24) =100100a(25) =104104a(26) =108108a(27) =112112a(28) =116116a(29) =120120