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 A366142 #84 Mar 11 2025 07:12:58 %S A366142 1,2,3,4,5,7,8,9,11,12,16,17,18,19,20,23,25,27,28,31,32,36,37,44,45, %T A366142 48,49,50,53,59,61,63,64,67,68,71,72,75,76,80,81,83,92,97,98,99,100, %U A366142 103,107,108,112,121,124,125,127,128,131,144,147,148,151,153,157,162,169,171,175,176,180 %N A366142 Matula-Goebel numbers of rooted trees which are symmetrical about a straight line passing through the root. %C A366142 The Matula-Goebel number of a tree is Product prime(k_i), where the k_i are the Matula-Goebel numbers of the child subtrees of the root. %C A366142 A tree is symmetric about a line iff the root has 2 copies of each child subtree (one each side of the line), and an optional "middle" child subtree on the line and in turn symmetric too. %H A366142 Ramzan Guekhaev, <a href="https://docs.google.com/document/d/1Jw9gCVk9xGeHbuXoEKlj_X2lCXFFZLHjA8zVrGDSpnU/edit?usp=sharing">Flowery numbers.docx</a>. %H A366142 Ramzan Guekhaev, <a href="https://docs.google.com/document/d/1b-OuKxIZkuoftma9n6ad3Y5BrElAwcM6uxK7SuKk2iM/edit?usp=drivesdk">Table for n, a(n) for n = 1..455</a> %H A366142 <a href="/index/Mat#matula">Index entries for sequences related to Matula-Goebel numbers</a> %F A366142 a(1) = 1; k > 1 is a term iff (k/p^2 is a term for some p) OR (k = prime(j) where j is a term). %e A366142 12 is a term since it's the Matula-Goebel number of the following tree which is, per the layout shown, symmetric about the vertical. %e A366142 (*) %e A366142 | %e A366142 (*) (*) (*) %e A366142 \ | / %e A366142 \ | / %e A366142 (*) root %Y A366142 Cf. A000040. %K A366142 nonn %O A366142 1,2 %A A366142 _Ramzan Guekhaev_ and _Loïc Apothéloz_, Sep 30 2023