Number of permutations of 2 copies of 1..n with all adjacent differences <= 1 in absolute value.

Terms

a(0) =1a(1) =1a(2) =6a(3) =12a(4) =26a(5) =48a(6) =86a(7) =148a(8) =250a(9) =416a(10) =686a(11) =1124a(12) =1834a(13) =2984a(14) =4846a(15) =7860a(16) =12738a(17) =20632a(18) =33406a(19) =54076a(20) =87522a(21) =141640a(22) =229206a(23) =370892a(24) =600146a(25) =971088a(26) =1571286a(27) =2542428a(28) =4113770a(29) =6656256