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 A124040 #22 Feb 07 2025 01:28:37
%S A124040 3,3,-1,8,-6,1,20,-24,9,-1,45,-84,50,-12,1,125,-275,225,-85,15,-1,320,
%T A124040 -864,900,-468,129,-18,1,845,-2639,3339,-2219,840,-182,21,-1,2205,
%U A124040 -7896,11756,-9528,4610,-1368,244,-24,1,5780,-23256,39825,-38121,22518,-8532,2079,-315,27,-1,15125,-67650,130975,-144660,101065,-46746,14525,-3000,395,-30,1
%N A124040 Triangle read by rows: characteristic polynomials of certain matrices, see Mathematica program.
%C A124040 Matrices:
%C A124040 1 X 1 : {{3}},
%C A124040 2 X 2 : {{3, 1}, {1, 3}},
%C A124040 3 X 3 : {{3, 1, 1}, {1, 3, 1}, {1, 1, 3}},
%C A124040 4 X 4 : {{3, 1, 0, 1}, {1, 3, 1, 0}, {0, 1, 3, 1}, {1, 0, 1, 3}},
%C A124040 5 X 5 : {{3, 1, 0, 0, 1}, {1, 3, 1, 0, 0}, {0, 1, 3, 1, 0}, {0, 0, 1, 3, 1}, {1, 0, 0, 1, 3}}.
%H A124040 G. C. Greubel, <a href="/A124040/b124040.txt">Rows n = 1..50 of the triangle, flattened</a>
%F A124040 From _G. C. Greubel_, Feb 03 2025: (Start)
%F A124040 T(n, 1) = A099921(n-1) + 3*[n=1] - 2*[n=2] + 3*[n=3].
%F A124040 T(n, 2) = -(n-1)*Fibonacci(2*n-2).
%F A124040 T(n, 3) = (1/10)*(n-1)*(2*(n-1)*Fibonacci(2*n-1) - (n+2)*Fibonacci(2*n-2)).
%F A124040 T(n, 4) = (1/150)*(n-1)*(18*(n-1)*Fibonacci(2*n-1) - (5*n^2 - n + 18)*Fibonacci(2*n-2)).
%F A124040 T(n, 5) = (1/600)*(n-1)*(2*(n-1)*(n^2-2*n+24)*Fibonacci(2*n-1) - (n^3+15*n^2 -10*n+48)*Fibonacci(2*n-2)).
%F A124040 T(n, n) = (-1)^(n-1) + 2*[n=1].
%F A124040 T(n, n-1) = 3*(-1)^n*(n-1).
%F A124040 T(n, n-2) = (1/2)*(-1)^(n+1)*(n-1)*(9*n-20) + [n=3].
%F A124040 T(n, n-3) = (3/2)*(-1)^n*(n-1)*(n-3)*(3*n-8) + 2*[n=4].
%F A124040 T(n, n-4) = (1/8)*(-1)^(n-1)*n*(n-3)*(27*n^2-117*n+130) - 2*[n=5].
%F A124040 T(n, n-5) = (3/40)*(-1)^n*(n-1)*(n-4)*(n-5)*(27*n^2-195*n+362) + 2*[n=6].
%F A124040 T(n, n-6) = (1/240)*(-1)^(n-1)*(n-1)*(n-5)*(n-6)(243*n^3-2997*n^2+12528*n -17752) -2*[n=7].
%F A124040 T(2*n-1, n) = 2*(-1)^(n-1)*A370280(n-1) + [n=1].
%F A124040 Sum_{k=1..n} T(n, k) = A010675(n-1) + 3*[n=1] -2*[n=2] +3*[n=3].
%F A124040 Sum_{k=1..floor((n+1)/2)} T(n-k+1, k) = A201630(n-2) - A201630(n-1) + (1/4)*[n=1] + (7/2)*[n=2] + 2*[n=3].
%F A124040 (End)
%e A124040 Triangle begins:
%e A124040 3;
%e A124040 3, -1;
%e A124040 8, -6, 1;
%e A124040 20, -24, 9, -1;
%e A124040 45, -84, 50, -12, 1;
%e A124040 125, -275, 225, -85, 15, -1;
%e A124040 320, -864, 900, -468, 129, -18, 1;
%e A124040 845, -2639, 3339, -2219, 840, -182, 21, -1;
%e A124040 2205, -7896, 11756, -9528, 4610, -1368, 244, -24, 1;
%e A124040 5780, -23256, 39825, -38121, 22518, -8532, 2079, -315, 27, -1;
%t A124040 T[n_, m_, d_]:= If[n==m, 3, If[n==m-1 || n==m+1, 1, If[(n==1 && m==d) || (n==d && m==1), 1, 0]]];
%t A124040 M[d_]:= Table[T[n, m, d], {n,d}, {m,d}];
%t A124040 Join[{M[1]}, Table[CoefficientList[Det[M[d] - x*IdentityMatrix[d]], x], {d,12}]]
%Y A124040 Cf. A010675, A099921, A201630, A370280.
%K A124040 uned,easy,tabl,sign
%O A124040 1,1
%A A124040 _Gary W. Adamson_ and _Roger L. Bagula_, Nov 04 2006