Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 1 local maximum.

A159715

Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 1 local maximum.

Terms

    a(0) =4a(1) =18a(2) =72a(3) =270a(4) =972a(5) =3402a(6) =11664a(7) =39366a(8) =131220a(9) =433026a(10) =1417176a(11) =4605822a(12) =14880348a(13) =47829690a(14) =153055008a(15) =487862838a(16) =1549681956a(17) =4907326194a(18) =15496819560a(19) =48814981614a(20) =153418513644a(21) =481176247338

External references