Number of permutations of length n which avoid the patterns 231, 51234.

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =41a(5) =119a(6) =336a(7) =927a(8) =2527a(9) =6870a(10) =18717a(11) =51155a(12) =140120a(13) =384147a(14) =1053147a(15) =2886182a(16) =7906953a(17) =21657679a(18) =59318944a(19) =162475127a(20) =445041287a(21) =1219069014a(22) =3339354501a(23) =9147387195a(24) =25057021032a(25) =68637206987a(26) =188013260835a(27) =515011570790