a(n) = n for n <= 2; for n > 2, a(n) = 2a(n-1) - a(n - floor( 1/2 + sqrt(2n) )).

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =5a(5) =8a(6) =13a(7) =23a(8) =41a(9) =74a(10) =135a(11) =257a(12) =491a(13) =941a(14) =1808a(15) =3481a(16) =6827a(17) =13397a(18) =26303a(19) =51665a(20) =101522a(21) =199563a(22) =395645a(23) =784463a(24) =1555529a(25) =3084755a(26) =6117845a(27) =12134168a(28) =24068773a(29) =47937983