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a(n) = Sum_{i=1..floor((n+1)/4)} a(2*i-1) * a(n-2*i+1), with a(1)=2, a(2)=1, and a(3)=3.
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%I #6 Jul 21 2019 18:52:25

%S 2,1,3,6,12,24,66,168,408,1014,2820,7656,20370,54096,154416,430710,

%T 1182504,3258816,9377922,26651328,74941608,210709446,612566616,

%U 1760983512,5017273554,14308403856,41833393896,121321861350,349444725492,1006135834560

%N a(n) = Sum_{i=1..floor((n+1)/4)} a(2*i-1) * a(n-2*i+1), with a(1)=2, a(2)=1, and a(3)=3.

%K nonn

%O 1,1

%A _Clark Kimberling_