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%I
%S 1,2,6,24,120,718,4884,35546,264590,1969912,14560996,106781758,
%T 778073440,5643649364,40811356112,294554236798,2123419108962,
%U 15296729408614,110149122090326,792974948890068,5707931771206042
%N The number of permutations of length n that can be sorted by 4 pop stacks in parallel.
%H R. Smith and V. Vatter, <a href="https://www.semanticscholar.org/paper/The-enumeration-of-permutations-sortable-by-pop-in-Smith-Vatter/653c589cffdddc1f6964e1741574742cc7b00546">The enumeration of permutations sortable by pop stacks in parallel</a>
%F G.f.: (1584*x^11-14016*x^10+45432*x^9-79454*x^8+86562*x^7-62624*x^6 +30887*x^5-10411* x^4+2352*x^3-339*x^2+28*x-1) / (7920*x^11-40116*x^10 +96324*x^9-139728*x^8+133468 *x^7-87278*x^6+39670*x^5-12495*x^4 +2666*x^3-366*x^2+29*x-1).
%K nonn
%O 1,2
%A _Vincent Vatter_, Aug 29 2009
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