1,2

M. Albert, M. Atkinson, and V. Vatter, Classes of permutations with infinitely many well-behaved simple permutations

G.f. f satisfies (x^3-2*x^2+x)*f^4+(4*x^3-9*x^2+6*x-1)*f^3+(6*x^3-12*x^2+7*x-1)*f^2+(4*x^3-5*x^2+x)*f+x^3 = 0.

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

Sequence in context: A165536 A032351 A165537 this_sequence A165539 A109033 A049135

Adjacent sequences: A165535 A165536 A165537 this_sequence A165539 A165540 A165541

nonn

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