The number of permutations of length n in a particular geometric grid class.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =244a(6) =786a(7) =2458a(8) =7510a(9) =22527a(10) =66579a(11) =194408a(12) =561988a(13) =1610900a(14) =4584426a(15) =12966225a(16) =36476173a(17) =102132412a(18) =284785878a(19) =791182318a(20) =2190833086a(21) =6048706947a(22) =16655647911a(23) =45752451536a(24) =125405039368a(25) =343040546984a(26) =936651104466