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 A188626 #39 Sep 08 2022 08:45:56
%S A188626 1,3,7,16,37,86,199,456,1033,2314,5131,11276,24589,53262,114703,
%T A188626 245776,524305,1114130,2359315,4980756,10485781,22020118,46137367,
%U A188626 96469016,201326617,419430426,872415259,1811939356,3758096413
%N A188626 a(n) = n + (n-1)*2^(n-2).
%C A188626 Number of idempotent elements in IDT_n (Identity Difference Full Transformation Semigroup), denoted by E(IDT_n).
%H A188626 G. C. Greubel, <a href="/A188626/b188626.txt">Table of n, a(n) for n = 1..1000</a>
%H A188626 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (6,-13,12,-4).
%F A188626 a(n) = n + (n-1)*2^(n-2).
%F A188626 G.f. x*(1-3*x+2*x^2+x^3) / ( (2*x-1)^2*(x-1)^2 ). - _R. J. Mathar_, Apr 14 2011
%F A188626 E.g.f.: (2*exp(2*x)*x + 4*exp(x)*x - exp(2*x) + 1)/4. - _Stefano Spezia_, Dec 23 2021
%e A188626 For n = 4, #E(IDT_n)= 16.
%t A188626 Table[n + (n-1)*2^(n-2), {n,1,50}] (* _G. C. Greubel_, Nov 01 2018 *)
%t A188626 LinearRecurrence[{6,-13,12,-4},{1,3,7,16},40] (* _Harvey P. Dale_, Dec 31 2018 *)
%o A188626 (PARI) a(n) = n+(n-1)*2^(n-2) \\ _Michel Marcus_, Jun 29 2013
%o A188626 (Magma) [n + (n-1)*2^(n-2): n in [1..50]]; // _G. C. Greubel_, Nov 01 2018
%K A188626 nonn,easy
%O A188626 1,2
%A A188626 _Adeniji, Adenike_ & Makanjuola, Samuel, Apr 14 2011