Number of permutations of length n containing exactly 1 occurrence of 123 and 2 occurrences of 132.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =8a(5) =26a(6) =79a(7) =232a(8) =664a(9) =1856a(10) =5072a(11) =13568a(12) =35584a(13) =91648a(14) =232192a(15) =579584a(16) =1427456a(17) =3473408a(18) =8359936a(19) =19922944a(20) =47054848a(21) =110231552a(22) =256311296a(23) =591921152a(24) =1358430208a(25) =3099590656a(26) =7034896384a(27) =15888023552a(28) =35718692864a(29) =79960211456