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 A206601 #11 Aug 12 2013 18:51:52 %S A206601 0,2,26,728,59048,14348906,10460353202,22876792454960, %T A206601 150094635296999120,2954312706550833698642, %U A206601 174449211009120179071170506,30903154382632612361920641803528,16423203268260658146231467800709255288,26183890704263137277674192438430182020124346 %N A206601 3^(n(n+1)/2) - 1. %C A206601 There are n cities located on the vertices of a convex n-gon and 2 types of communication lines available. Any city can be connected to any other by only one communication line (that can be of any type). A network exists if at least 2 cities are connected by a communication line. The sequence shows how many different networks a(n) can be built. In general, if the number of communication-line types is c, then a(n) = (c+1)^(n(n+1)/2)-1. Thus other sequences of this type can be generated. %F A206601 a(n) = (3^A000217) - 1. %F A206601 a(n) = A047656(n+1) - 1. - _Omar E. Pol_, Feb 18 2012 %e A206601 In the case of 2 different types of communication lines and 4 cities, the number of different networks (connecting at least 2 cities) is 728. %Y A206601 Cf. A000217, A126883. %K A206601 easy,nonn %O A206601 0,2 %A A206601 _Ivan N. Ianakiev_, Feb 10 2012