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 A319321 #10 Oct 27 2018 03:27:54 %S A319321 1,4,20,108,600,3368,18968,106906,602532,3395402,19131460,107788900, %T A319321 607274848 %N A319321 a(n) is the number of d+/d- diagonally convex polyplets (polyominoes which need only touch at corners) with n cells. %C A319321 A polyplet is d+ [d-] diagonally convex if the intersection of its interior with any line of slope 1 [-1] through the centers of the cells is connected. %H A319321 Adam Gruber, <a href="https://devweb3000.cis.strath.ac.uk/~xpb16190/LatticeAnimals/LatticeAnimals.jar">Java program to count lattice animals</a>. %e A319321 The only tetraplets that are not d+ diagonally convex are two animals consisting of a horizontal domino and a vertical domino joined at a corner. So a(4) = A006770(4) - 2 = 108. %Y A319321 Cf. A006770 (fixed polyplets), A187077 (row convex polyplets), A187276 (d+/d- diagonally convex polyominoes). %K A319321 nonn,more %O A319321 1,2 %A A319321 _David Bevan_, Sep 27 2018