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a(n)=4^C(n,2)*(4^n-1)/3.
1

%I #4 Aug 29 2015 15:29:47

%S 0,1,20,1344,348160,357564416,1465657589760,24017731997138944,

%T 1574098141758535761920,412645105639632468417970176,

%U 432690992231222540584116394393600,1814838857553600260569028862094736359424,30447948962887853884238295543591323273401466880

%N a(n)=4^C(n,2)*(4^n-1)/3.

%C -a(n) is the Hankel transform of A127846, the series reversion of x/(1+5x+4x^2). Note that (4^n-1)/3 has g.f. x/(1-5x+4x^2).

%t Table[4^Binomial[n,2] (4^n-1)/3,{n,0,20}] (* _Harvey P. Dale_, Aug 29 2015 *)

%K easy,nonn

%O 0,3

%A _Paul Barry_, Feb 02 2007

%E More terms from _Harvey P. Dale_, Aug 29 2015