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A322599 a(n) is the number of unlabeled rank-3 graded lattices with 4 coatoms and n atoms. 4

%I

%S 1,4,13,34,68,121,197,299,432,600,806,1055,1352,1698,2100,2561,3085,

%T 3675,4338,5074,5891,6790,7777,8854,10029,11300,12677,14160,15756,

%U 17465,19297,21249,23332,25544,27894,30381,33016,35794,38728,41815,45065

%N a(n) is the number of unlabeled rank-3 graded lattices with 4 coatoms and n atoms.

%H Jukka Kohonen, <a href="/A322599/b322599.txt">Table of n, a(n) for n = 1..1000</a>

%H J. Kohonen, <a href="http://arxiv.org/abs/1804.03679">Counting graded lattices of rank three that have few coatoms</a>, arXiv:1804.03679 [math.CO] preprint (2018).

%F a(n) = (97/144)n^3 - (5/6)n^2 + [44/48, 47/48]n + [0, 13, 8, -45, 40, -19, 0, -5, 8, -27, 40, -37]/72. The value of the first bracket depends on whether n is even or odd. The value of the second bracket depends on whether (n mod 12) is 0, 1, 2, ..., 11.

%F Conjectures from _Colin Barker_, Dec 19 2018: (Start)

%F G.f.: x*(1 + 3*x + 8*x^2 + 17*x^3 + 21*x^4 + 21*x^5 + 16*x^6 + 7*x^7 + 3*x^8) / ((1 - x)^4*(1 + x)^2*(1 + x^2)*(1 + x + x^2)).

%F a(n) = a(n-1) + a(n-2) - 2*a(n-5) + a(n-8) + a(n-9) - a(n-10) for n>10.

%F (End)

%e a(2)=4: These are the four lattices.

%e __o__ __o__ __o__ __o__

%e / / \ \ / / \ \ / / \ \ / / \ \

%e o o o o o o o o o o o o o o o o

%e \_\ /_/| \|/ \| \|/ | |/ \|

%e o o o o o o o o

%e \ / \ / \ / \_ _/

%e o o o o

%Y Fourth row of A300260.

%Y Adjacent rows are A322598, A322600.

%K nonn,easy

%O 1,2

%A _Jukka Kohonen_, Dec 19 2018

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Last modified May 19 17:48 EDT 2019. Contains 323395 sequences. (Running on oeis4.)