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 A364485 #6 Jul 29 2023 05:01:38
%S A364485 1,2,4,9,23,59,155,418,1136,3122,8663,24201,68059,192471,546899,
%T A364485 1560511,4469000,12839642,36995629,106875531,309477998,898075778,
%U A364485 2611239508,7606064348,22191694916,64845964156,189752911736,555985221037,1631053277370,4790356866561
%N A364485 Number of fixed triangular polyominoes with 2n cells of the regular tiling with Schläfli symbol {3,6} that have a common axis of symmetry coincident with cell edges.
%C A364485 Half of the cells lie on each side of the axis of symmetry, so there must be an even number. This is one of three sequences used to calculate A030223, the number of achiral polyominoes for this tiling. Two fixed polyominoes are identical only if one is a translation of the other.
%F A364485 a(n) = 2*A030223(2n) - A364486(2n) - A364487(2n).
%e A364485 These are halves above the axis of symmetry of the polyominoes for n<4. The bottom edge of each is the axis of symmetry.
%e A364485 /\ /\
%e A364485 ____ ____ ________ ____ /__\ /__\
%e A364485 /\ /\ / \ /\ \ /\ / /\ /\ /\ / \ /\
%e A364485 /__\ /__\/ \/__\ \/__\/ /__\/__\ /__\/ \/__\
%Y A364485 Cf. A030223, A364486, A364487.
%K A364485 nonn
%O A364485 1,2
%A A364485 _Robert A. Russell_, Jul 26 2023