The number of permutations of length n sortable by 2 reversals.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =63a(6) =145a(7) =288a(8) =516a(9) =857a(10) =1343a(11) =2010a(12) =2898a(13) =4051a(14) =5517a(15) =7348a(16) =9600a(17) =12333a(18) =15611a(19) =19502a(20) =24078a(21) =29415a(22) =35593a(23) =42696a(24) =50812a(25) =60033a(26) =70455a(27) =82178a(28) =95306a(29) =109947