The number of length-n permutations avoiding the patterns 1234, 1324 and 2143.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =21a(5) =75a(6) =268a(7) =958a(8) =3425a(9) =12245a(10) =43778a(11) =156514a(12) =559565a(13) =2000543a(14) =7152292a(15) =25570698a(16) =91419729a(17) =326841561a(18) =1168515890a(19) =4177649198a(20) =14935828405a(21) =53398205443a(22) =190907947468a(23) =682529386598