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 A168604 #45 Sep 08 2022 08:45:49
%S A168604 1,3,7,15,31,63,127,255,511,1023,2047,4095,8191,16383,32767,65535,
%T A168604 131071,262143,524287,1048575,2097151,4194303,8388607,16777215,
%U A168604 33554431,67108863,134217727,268435455,536870911,1073741823,2147483647,4294967295,8589934591
%N A168604 a(n) = 2^(n-2) - 1.
%C A168604 Number of ways of partitioning the multiset {1,1,1,2,3,...,n-2} into exactly two nonempty parts.
%C A168604 An elephant sequence, see A175655. For the central square six A[5] vectors, with decimal values between 26 and 176, lead to this sequence. For the corner squares these vectors lead to the companion sequence A000325 (without the first leading 1). - _Johannes W. Meijer_, Aug 15 2010
%H A168604 Vincenzo Librandi, <a href="/A168604/b168604.txt">Table of n, a(n) for n = 3..1000</a>
%H A168604 M. Griffiths, I. Mezo, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Griffiths/griffiths11.html">A generalization of Stirling Numbers of the Second Kind via a special multiset</a>, JIS 13 (2010) #10.2.5
%H A168604 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2).
%F A168604 E.g.f.: 2*exp(2*x)-exp(x).
%F A168604 a(n) = A000225(n-2).
%F A168604 G.f.: x^3/((1-x)*(1-2*x))
%F A168604 a(n) = A126646(n-3). - _R. J. Mathar_, Dec 11 2009
%F A168604 a(n) = 3*a(n-1) - 2*a(n-2). - _Arkadiusz Wesolowski_, Jun 14 2013
%F A168604 a(n) = A000918(n-2) + 1. - _Miquel Cerda_, Aug 09 2016
%e A168604 The partitions of {1,1,1,2,3} into exactly two nonempty parts are {{1},{1,1,2,3}}, {{2},{1,1,1,3}}, {{3},{1,1,1,2}}, {{1,1},{1,2,3}}, {{1,2},{1,1,3}}, {{1,3},{1,1,2}} and {{2,3},{1,1,1}}.
%t A168604 f4[n_] := 2^(n - 2) - 1; Table[f4[n], {n, 3, 30}]
%t A168604 LinearRecurrence[{3,-2},{1,3},40] (* _Harvey P. Dale_, Oct 20 2013 *)
%o A168604 (Magma) [2^(n-2)-1 : n in [3..35]]; // _Vincenzo Librandi_, May 13 2011
%o A168604 (PARI) a(n)=2^(n-2)-1 \\ _Charles R Greathouse IV_, Oct 07 2015
%Y A168604 The number of ways of partitioning the multiset {1, 1, 1, 2, 3, ..., n-1} into exactly three and four nonempty parts are given in A168605 and A168606, respectively.
%K A168604 nonn,easy
%O A168604 3,2
%A A168604 _Martin Griffiths_, Dec 01 2009