1,2

There are 22 permutations of length 4 which avoid these two patterns, so a(4)=22.

Sequence in context: A006318 A155069 A103137 this_sequence A053617 A089449 A150271

Adjacent sequences: A165543 A165544 A165545 this_sequence A165547 A165548 A165549

nonn

Vince Vatter (vatter(AT)gmail.com), Sep 21 2009