a(n) = ceiling(sqrt(4*10^n)).

Terms

a(0) =2a(1) =7a(2) =20a(3) =64a(4) =200a(5) =633a(6) =2000a(7) =6325a(8) =20000a(9) =63246a(10) =200000a(11) =632456a(12) =2000000a(13) =6324556a(14) =20000000a(15) =63245554a(16) =200000000a(17) =632455533a(18) =2000000000a(19) =6324555321a(20) =20000000000a(21) =63245553204a(22) =200000000000a(23) =632455532034