Number of permutations of length n sortable by 4 passes through a pop-stack.

A293775

Number of permutations of length n sortable by 4 passes through a pop-stack.

Terms

    a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =120a(6) =565a(7) =2473a(8) =10468a(9) =44148a(10) =187363a(11) =799605a(12) =3418967a(13) =14615713a(14) =62439735a(15) =266643093a(16) =1138577340a(17) =4862025964a(18) =20763336212a(19) =88672294260a(20) =378685960241

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