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 A377756 #29 Dec 03 2024 12:41:14 %S A377756 1,1,3,6,18,55,169,477,1245,2750,5380,8989,12674,14741,13928,10297, %T A377756 6185,2910,1012,289,69,12,2 %N A377756 Number of free hexagonal polyominoes with n cells with at most 3 collinear cell centers on any line in the plane. %C A377756 a(n) is the number of connected planer graphs with n nodes, where the nodes lie on a triangular lattice grid and no more than 3 nodes are collinear over the underlying plane. %C A377756 a(n) is the sum of columns 1-3 in A378015, the n-th term = Sum(T(n,k)) for k<=3. %H A377756 Dave Budd, <a href="https://github.com/daveisagit/oeis/blob/main/hex_grid/connected_nodes.py">Collinear cells in hexagon polyominoes</a> %H A377756 Dave Budd, <a href="https://github.com/daveisagit/collinear-polyominoes/blob/main/src/A377756.py">Optimized version for exhaustive proof of A377756</a> %e A377756 For n=23, the 2 hexagon polyominoes are: %e A377756 @ @ @ %e A377756 @ @ @ %e A377756 @ @ @ @ @ %e A377756 @ @ @ @ @ @ %e A377756 @ @ @ @ @ @ %e A377756 @ @ @ @ @ @ %e A377756 @ @ @ @ @ @ %e A377756 @ @ @ @ @ @ %e A377756 @ @ @ @ @ %o A377756 (Python) # See links %Y A377756 Cf. A000228, A378015. %K A377756 nonn,fini,full %O A377756 1,3 %A A377756 _Dave Budd_, Nov 06 2024