This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A107240 #24 Feb 16 2025 08:32:57
%S A107240 1,2,3,12,37,118,407,1368,4617,15642,52891,178916,605325,2047726,
%T A107240 6927407,23435376,79281105,268206130,907335091,3069492092,10384017717,
%U A107240 35128880742,118840150983,402033352264,1360069089113,4601080768074
%N A107240 Sum of squares of first n tribonacci numbers (A000213).
%H A107240 Harvey P. Dale, <a href="/A107240/b107240.txt">Table of n, a(n) for n = 1..1000</a>
%H A107240 M. Feinberg, <a href="https://web.archive.org/web/2024*/https://www.fq.math.ca/Scanned/1-3/feinberg.pdf">Fibonacci-Tribonacci</a>, Fib. Quart. 1(3) (1963), 71-74.
%H A107240 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TribonacciNumber.html">Tribonacci Number</a>.
%H A107240 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (3, 1, 3, -7, 1, -1, 1).
%F A107240 a(n) = Sum_{i=1..n} A000213(i)^2.
%F A107240 a(n)= 3*a(n-1) +a(n-2) +3*a(n-3) -7*a(n-4) +a(n-5) -a(n-6) +a(n-7). G.f.: (x^3-x^2+3*x-1)*(1+x)^2/((x-1)*(x^3+x^2+3*x-1)*(x^3-x^2-x-1)). - _R. J. Mathar_, Aug 11 2009
%e A107240 a(6) = 1^2 + 1^2 + 1^2 + 3^2 + 5^2 + 9^2 = 118.
%t A107240 Accumulate[LinearRecurrence[{1,1,1},{1,1,1},30]^2] (* _Harvey P. Dale_, Nov 11 2011 *)
%t A107240 LinearRecurrence[{3, 1, 3, -7, 1, -1, 1},{1, 2, 3, 12, 37, 118, 407},26] (* _Ray Chandler_, Aug 02 2015 *)
%Y A107240 Cf. A000213, A001654, A107239, A107241-A107247.
%K A107240 easy,nonn
%O A107240 1,2
%A A107240 _Jonathan Vos Post_, May 14 2005