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 A254131 #19 Nov 05 2015 14:31:32
%S A254131 0,1,-1,2,2,-2,41,13,3,-3,30,90,24,4,-4,501,209,139,35,5,-5,322,1102,
%T A254131 388,188,46,6,-6,5041,2253,1703,567,237,57,7,-7,3110,11090,4184,2304,
%U A254131 746,286,68,8,-8,47501,21769,17139,6115,2905,925,335,79,9,-9
%N A254131 Rectangular array A read by upward antidiagonals in which the entry in row n and column k is defined by A(n,k) = A254067(n,k) - A257499(n,k), n,k >= 1.
%F A254131 A(n,k) = (1 + (3^n - 2^(n + 1))*(6*k - 3 + 2*(-1)^n))/6, n,k >= 1.
%e A254131 A begins:
%e A254131 . 0 -1 -2 -3 -4 -5 -6 -7 -8 -9
%e A254131 . 1 2 3 4 5 6 7 8 9 10
%e A254131 . 2 13 24 35 46 57 68 79 90 101
%e A254131 . 41 90 139 188 237 286 335 384 433 482
%e A254131 . 30 209 388 567 746 925 1104 1283 1462 1641
%e A254131 . 501 1102 1703 2304 2905 3506 4107 4708 5309 5910
%e A254131 . 322 2253 4184 6115 8046 9977 11908 13839 15770 17701
%e A254131 . 5041 11090 17139 23188 29237 35286 41335 47384 53433 59482
%e A254131 . 3110 21769 40428 59087 77746 96405 115064 133723 152382 171041
%e A254131 . 47501 104502 161503 218504 275505 332506 389507 446508 503509 560510
%t A254131 (* Array: *)
%t A254131 A254131[n_, k_] := (1 + (3^n - 2^(n + 1))*(6*k - 3 + 2*(-1)^n))/6; Grid[Table[A254131[n, k], {n, 10}, {k, 10}]]
%t A254131 (* Array antidiagonals flattened: *)
%t A254131 Flatten[Table[(1 + (3^(n - k + 1) - 2^(n - k + 2))*(6*k - 3 + 2*(-1)^(n - k + 1)))/6, {n, 10}, {k, n}]]
%K A254131 sign,tabl
%O A254131 1,4
%A A254131 _L. Edson Jeffery_, May 03 2015