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%I #16 Mar 13 2021 16:23:24
%S 1,35,189,559,1241,2331,3925,6119,9009,12691,17261,22815,29449,37259,
%T 46341,56791,68705,82179,97309,114191,132921,153595,176309,201159,
%U 228241,257651,289485,323839,360809,400491,442981,488375,536769
%N a(n) = 16*n^3 - 36*n^2 + 30*n - 9.
%C The n-th term of A155883 (hexagonal bifrustum numbers) has a hexagonal pyramid of [n - 1] set on each of its two hexagonal faces.
%C The digital roots run recursively 1, 8, 9.
%C The sum of the first n consecutive terms is the square of the n-th hexagonal number.
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-6,4,-1).
%F a(n) = 16*n^3 - 36*n^2 + 30*n - 9.
%F a(n) = A155883(n) + 2*A000578(n-1).
%F G.f.: x*(1 + 31*x + 55*x^2 + 9*x^3)/(1 - x)^4. - _Stefano Spezia_, Feb 04 2021
%e For n = 3 the solution is 173 + 8 + 8 = 189.
%Y Cf. A000384, A000578, A155883.
%K nonn,easy
%O 1,2
%A _David Z. Crookes_, Feb 03 2021
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