Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 2 adjacent element pairs in decreasing order.

A151583

Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 2 adjacent element pairs in decreasing order.

Terms

    a(0) =0a(1) =2a(2) =45a(3) =260a(4) =1115a(5) =4230a(6) =15113a(7) =52232a(8) =176823a(9) =590090a(10) =1948133a(11) =6376716a(12) =20725523a(13) =66960782a(14) =215232705a(15) =688746512a(16) =2195381615a(17) =6973567506a(18) =22082966429a(19) =69735686420a(20) =219667415499a(21) =690383309462

External references