

A216040


Number of permutations sortable using two parallel stacks.


4



1, 1, 2, 6, 23, 103, 513, 2760, 15741, 93944, 581303, 3704045, 24180340, 161082639, 1091681427, 7508269793, 52302594344, 368422746908, 2620789110712, 18806093326963, 136000505625886, 990406677136685, 7258100272108212
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OFFSET

0,3


LINKS



EXAMPLE

Up to n = 4, the only permutation that can't be sorted is 2341. This fails because after moving 2 to one stack, you must move 3 to the other stack, and now the 4 will block either the 2 or the 3. (If you use a doubleended queue instead of two stacks, then this permutation becomes sortable; cf. A182216.)


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



