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.
As an irregular triangle: 1; 2; 4, 3; 9, 1, 5, 4, 3; 8, 6, 5, 11, 10, 10, 6, 6, 9, 5, 2, 4; ... For n = 5, the L tetromino, whose binary code is A246521(5+1) = 15, can be extended to 9 different free pentominoes, so a(5) = 9. (All possible ways to add one cell lead to different pentominoes.) For n = 6, the square tetromino, whose binary code is A246521(6+1) = 23, can only be extended to the P pentomino by adding one cell, so a(6) = 1.
For n <= 3, any one polyomino with n cells is enough to construct the others (if any) by moving one cell, so a(n) = 1. For n = 4, either the L or the T tetromino suffices to construct the other four, so a(4) = 1. Below are examples of sets of a(n) polyominoes that are sufficient to construct all other polyominoes with n cells, 5 <= n <= 7: _ | | _ | |_ | |_ | _| | | |_| |_ _| _ | | _ | | | | _ _ | | | |_ _| _| | |_ | | | | |_ _| |_ _| |_ _| _ | | _ | | _ _ _ | | _ _ | | | |_ | |_ | | | |_ _ _| | | | | |_ | _| |_ | _| |_ _| _| _| _ _| _ _| |_ | _| | |_ _ _| |_ | _| | _| | _| | _ _ _| |_| |_ _ _| |_ _ _| |_ _| |_| |_| |_|
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