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 A144978 #3 Mar 31 2012 14:40:18 %S A144978 1,1,2,4,7,14,27,56,116,253,557,1272,2948,6998,16856,41306,102449, %T A144978 257294,652566,1670679,4311109,11206278,29316294,77144518,204072054, %U A144978 542446974,1448230644,3882179984,10445521740,28202158173,76387639678 %N A144978 A005195(n) - A005195(n-2). %C A144978 a(n) is the number of forests with n unlabeled nodes without trees of order 2. This follows from the fact that for n>=2 A005195(n-2) counts the forests of order n with one or more trees of order 2. %C A144978 The unique forest of order 1 is an isolated vertex, so a(1)=1. For n>=2, a(n) - a(n-1) counts forests of order n with trees of order >=3. %C A144978 A005195(n) - A005195(n-3) counts forests of order n without trees of order 3. %e A144978 a(5) = 7 since the only options are: 3 trees of order 5; 2 forests composed by trees of orders 4 and l; one forest with trees of orders [3 1 1]; and one forest with five isolated nodes. %Y A144978 Cf. A005195, A000055(trees). %K A144978 nonn %O A144978 1,3 %A A144978 _Washington Bomfim_, Sep 28 2008