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 A068605 #29 Jun 21 2025 11:14:44
%S A068605 2,18,84,300,930,2646,7112,18360,45990,112530,270204,638820,1490762,
%T A068605 3440430,7864080,17825520,40107726,89652906,199229060,440401500,
%U A068605 968883762,2122317318,4630511064,10066329000,21810380150,47110421826,101468601612,217969589460,467077692570
%N A068605 Number of functions from [1,2,...,n] to [1,2,...,n] such that the image contains exactly two elements.
%C A068605 The sequence is the column corresponding to k=2 in A090657. - _Geoffrey Critzer_, Mar 06 2009
%H A068605 Stefano Spezia, <a href="/A068605/b068605.txt">Table of n, a(n) for n = 2..3200</a>
%H A068605 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (9,-33,63,-66,36,-8).
%F A068605 a(n) = C(n, 2) * (2^n - 2).
%F A068605 O.g.f.: (4x^2/(1-2x)^3) - (2x^2/(1-x)^3). - _Geoffrey Critzer_, Mar 06 2009
%F A068605 E.g.f.: exp(x)*(2*exp(x) - 1)*x^2. - _Stefano Spezia_, May 06 2023
%t A068605 Table[ Binomial[n, 2]*(2^n - 2), {n, 2, 30}]
%o A068605 (Python)
%o A068605 def A068605(n): return n*(n-1)*((1<<n-1)-1) # _Chai Wah Wu_, Jun 20 2025
%Y A068605 Cf. A000918, A090657, A161680.
%K A068605 nonn,easy
%O A068605 2,1
%A A068605 Sharon Sela (sharonsela(AT)hotmail.com), Mar 29 2002
%E A068605 Edited and extended by _Robert G. Wilson v_, Apr 17 2002