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Number of set partitions of [n] with alternating parity of elements and exactly three blocks.
2

%I #10 Jun 10 2018 13:28:58

%S 1,3,7,14,30,57,119,224,460,867,1761,3338,6734,12861,25843,49748,

%T 99744,193431,387365,756062,1513138,2969265,5940567,11708072,23420228,

%U 46317195,92642569,183707186,367430742,730111269,1460255291,2906227196,5812519912,11582124159

%N Number of set partitions of [n] with alternating parity of elements and exactly three blocks.

%H Alois P. Heinz, <a href="/A305777/b305777.txt">Table of n, a(n) for n = 3..1000</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Partition_of_a_set">Partition of a set</a>

%H <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (3,4,-18,1,33,-16,-18,12)

%F G.f.: (10*x^6 -x^5 -13*x^4 +x^3 +6*x^2-1) *x^3 / ((2*x-1) *(x+1) *(3*x^2-1) *(2*x^2-1) *(x-1)^2).

%e a(5) = 7: 123|4|5, 12|34|5, 12|3|45, 1|234|5, 145|2|3, 1|2|345, 1|23|45.

%Y Column k=3 of A274581.

%K nonn,easy

%O 3,2

%A _Alois P. Heinz_, Jun 10 2018