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 A084615 #13 Mar 25 2023 08:20:24
%S A084615 1,5,23,99,401,1525,6345,27331,122083,520805,2117293,8301441,34517395,
%T A084615 147850771,628707981,2675100397,10920387779,43701876543,180872758207,
%U A084615 769658883325,3243501133481,13617178197183,56148348498199
%N A084615 a(n) = sum of absolute values of coefficients of (1+x-3x^2)^n.
%H A084615 G. C. Greubel, <a href="/A084615/b084615.txt">Table of n, a(n) for n = 0..1000</a>
%F A084615 a(n) = Sum_{k=0..2*n} abs( Sum_{j=0..k} binomial(n,k-j)*binomial(k-j,j)*(-3)^j ). - _G. C. Greubel_, Mar 25 2023
%t A084615 Table[Total[Abs[CoefficientList[Expand[(1+x-3x^2)^n],x]]],{n,0,30}] (* _Harvey P. Dale_, Mar 26 2013 *)
%o A084615 (PARI) for(n=0,40,S=0; for(k=0,2*n,t=polcoeff((1+x-3*x^2)^n,k,x); S=S+abs(t)); print1(S","))
%o A084615 (Magma)
%o A084615 A084614:= func< n,k | (&+[Binomial(n, k-j)*Binomial(k-j, j)*(-3)^j: j in [0..k]]) >;
%o A084615 [(&+[Abs(A084614(n,k)): k in [0..2*n]]): n in [0..50]]; // _G. C. Greubel_, Mar 25 2023
%o A084615 (SageMath)
%o A084615 @CachedFunction
%o A084615 def A084614(n,k): return sum(binomial(n,k-j)*binomial(k-j,j)*(-3)^j for j in range(k+1))
%o A084615 def A084615(n): return sum(abs(A084614(n,k)) for k in range(2*n+1))
%o A084615 [A084615(n) for n in range(50)] # _G. C. Greubel_, Mar 25 2023
%Y A084615 Cf. A002426, A084600 - A084614.
%K A084615 nonn
%O A084615 0,2
%A A084615 _Paul D. Hanna_, Jun 01 2003