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 A381123 #18 Feb 21 2025 12:24:56 %S A381123 1,0,2,4,12,28,83,213,608,1664,4703,13173,37412,105995,302301,862794, %T A381123 2470631,7084425,20357121,58573788,168789684,486964114,1406549550, %U A381123 4066751083,11769363663,34090076148,98820914068,286672673725,832183340955,2417270306657,7025657374736,20430883575932,59444386613999,173039084438093 %N A381123 Number of unlabeled endofunctions on n points whose self-referencing elements are mapped from another element. %C A381123 Equivalently, the number of digraphs on n unlabeled nodes where each node has an out degree 1 and, if it is self-referencing, it is referenced from at least one other node. %H A381123 Alois P. Heinz, <a href="/A381123/b381123.txt">Table of n, a(n) for n = 0..1000</a> %F A381123 a(0) = 1; a(n) = A001372(n) - A001372(n - 1) for n > 0. %e A381123 For n = 2 one node must reference the other one, this one may reference itself or the first one. So a(2) = 2. %e A381123 For n = 3 there are 7 = A001372(3) endofunctions, but 3 = A001372(2) of them have at least 1 isolated element. So a(3) = 7 - 3 = 4. %Y A381123 Partial differences of A001372. %K A381123 nonn %O A381123 0,3 %A A381123 _Peter Dolland_, Feb 14 2025