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 A342536 #19 Mar 27 2021 01:20:52 %S A342536 1,1,3,4,10,17,36,65,126,227,419,743,1323,2295,3965 %N A342536 Number of self-avoiding polygons on the square lattice, of perimeter 2n, with the property that all the right-angles of the same orientation are contiguous. %C A342536 For any polygon, build a bracelet of black and white beads by following the border of the polygon in a clockwise direction, adding a black bead for each right-turning right angle, and a white bead for each left-turning right angle. The polygons counted by this sequence are those whose bracelets have all the black beads together and all the white beads together. %H A342536 John Mason, <a href="/A342536/a342536.pdf">Contiguously oriented saps</a> %e A342536 a(4)=3, as there are 3 self-avoiding polygons (SAPs) of perimeter 8 that satisfy the condition; these are the polygons corresponding to the strip and L-shaped trominoes, and the square tetromino. %Y A342536 Cf. A266549. %K A342536 nonn,more %O A342536 2,3 %A A342536 _John Mason_, _Allan C. Wechsler_, Mar 15 2021