a(n) = phi(2n+1).

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =6a(5) =10a(6) =12a(7) =8a(8) =16a(9) =18a(10) =12a(11) =22a(12) =20a(13) =18a(14) =28a(15) =30a(16) =20a(17) =24a(18) =36a(19) =24a(20) =40a(21) =42a(22) =24a(23) =46a(24) =42a(25) =32a(26) =52a(27) =40a(28) =36a(29) =58