Surround numbers of a length 2n zig-zag.

Terms

a(0) =161a(1) =257a(2) =373a(3) =509a(4) =665a(5) =841a(6) =1037a(7) =1253a(8) =1489a(9) =1745a(10) =2021a(11) =2317a(12) =2633a(13) =2969a(14) =3325a(15) =3701a(16) =4097a(17) =4513a(18) =4949a(19) =5405a(20) =5881a(21) =6377a(22) =6893a(23) =7429a(24) =7985a(25) =8561a(26) =9157a(27) =9773a(28) =10409a(29) =11065