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 A383020 #30 Apr 22 2025 08:03:56 %S A383020 8,7,10,10,12,11,12,12,11,14,13,12,11,13,12,11,10,14,13,15,17,16,15, %T A383020 17,16,15,16,19,18,18,17,16,16,15,14,17,16,15,15,14,13,13,12,15,19,18, %U A383020 17,16,18,19,20,22,21,22,21,23,23,22,21,24,23,22,24,23,24,23 %N A383020 G(n) is a graph constructed with nodes labelled with integers n through n+a(n). Edges are drawn between consecutive integers and between integers sharing the same largest prime factor. a(n) is the smallest integer for which G(n) is not planar. %H A383020 Pontus von Brömssen, <a href="/A383020/b383020.txt">Table of n, a(n) for n = 2..10000</a> %F A383020 a(n) >= a(n-1) - 1. - _Pontus von Brömssen_, Apr 22 2025 %e A383020 a(2) = 8: the graph G(2) has nodes labelled 2-10. Consecutive integers are connected by an edge. Also pairwise connected are: 3, 6, and 9 because they have 3 as the largest prime factor; 2, 4, and 8 because they have 2 as the largest prime factor; 5 and 10 because they have 5 as the largest prime factor. Nodes 2 and 10 are not connected because although 2 is a prime factor of 10, it is not the largest prime factor. This graph is non-planar. a(2) is larger than 7 because the nodes 2-9 make a planar graph. So a(2) = 8. %Y A383020 Cf. A006530. %K A383020 nonn %O A383020 2,1 %A A383020 _Gordon Hamilton_, Apr 20 2025 %E A383020 a(15) corrected and a(41)-a(67) added by _Pontus von Brömssen_, Apr 21 2025