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 A350513 #22 Feb 14 2022 00:18:19 %S A350513 3,21,648,15552,404595,10812528,290993472,7849029276,211855234563, %T A350513 5719493462076,154420985682648,4169319089851512,112571188876032435, %U A350513 3039418274650387848,82064259021919140432,2215734684425617523796,59824833697843168341123,1615270484817096465311316 %N A350513 Number of 2 X n rectangular Celtic knots (up to rotations and symmetries). %H A350513 Jessica Connor and Nick Ward, <a href="https://www.maths.ed.ac.uk/~v1ranick/knots/celtic.pdf">Celtic Knot Theory</a> %F A350513 a(n) = (3^(3*n-2) + 3^((3*n-2)/2) + 3^(2*n-1) + 3^(3*n/2))/4 for even n > 2; %F A350513 a(n) = (3^(3*n-2) + 3^((3*n-1)/2) + 3^(2*n-1) + 3^((3*n-1)/2))/4 for odd n. %F A350513 a(n) = (3^(3*n-2) + 3^floor((3*n-1)/2) + 3^(2*n-1) + 3^floor(3*n/2))/4 for n <> 2. %o A350513 (PARI) a(n) = if(n==2, 21, (3^(3*n-2) + 3^((3*n-1)\2) + 3^(2*n-1) + 3^(3*n\2))/4) \\ _Andrew Howroyd_, Jan 03 2022 %Y A350513 Cf. A032120 (1 X n Celtic knots). %K A350513 nonn %O A350513 1,1 %A A350513 _Philippe Gibone_, Jan 02 2022