Number of permutations of length n that avoid 4123, 1324, and 31524.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =86a(6) =343a(7) =1374a(8) =5497a(9) =21926a(10) =87176a(11) =345612a(12) =1366960a(13) =5396604a(14) =21275618a(15) =83793268a(16) =329791425a(17) =1297428774a(18) =5103039279a(19) =20069815052a(20) =78936685887a(21) =310509337760