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.
Triangle begins: [2] [3, 1] [7, 7, 8] [11, 5, 71, 3] [26, 9, 679, 77, 52] [41, 13, 769, 281, 17753, 29] [97, 47, 3713, 4271, 726433, 434657, 272] [153, 17, 8449, 2245, 33507, 167089, 46069729, 901] [362, 123, 81767, 8569, 24852386, 265721, 8118481057, 190818387, 73124] [571, 89, 93127, 18061, 20721019, 4213133, 4974089647, 1031151241, 1234496016491, 89893] ...
a(0) = 1, the empty sandpile on an empty grid. a(1) = 4, all 4 sandpiles on a 1 X 1 grid are fully symmetric. a(2) = 2 fully symmetric recurrent sandpiles on an 2 X 2 grid are the all-2 and all-3 sandpiles. Among A348566(4, 4) = 36 sandpiles with horizontal and vertical symmetry, only a(4) = 12 also have diagonal symmetries: 3333 1331 1331 3223 3223 3333 2222 0330 2332 2332 0330 2222 3333 3223 3333 2222 2332 3223 2222 3223 3223 3333 3333 2332 3333 3223 3333 2222 2332 3223 2222 3223 3223 3333 3333 2332 3333 1331 1331 3223 3223 3333 2222 0330 2332 2332 0330 2222