a(n) = Sum_{k=1..n} floor((2*n-1)/(2*k+1)).

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =8a(5) =10a(6) =14a(7) =15a(8) =18a(9) =22a(10) =24a(11) =27a(12) =31a(13) =33a(14) =37a(15) =40a(16) =44a(17) =47a(18) =51a(19) =53a(20) =57a(21) =63a(22) =65a(23) =68a(24) =73a(25) =75a(26) =81a(27) =85a(28) =87a(29) =91