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 A075921 #14 Dec 27 2017 01:34:05
%S A075921 1,21,343,5145,74431,1058841,14941423,210003465,2945813311,
%T A075921 41281739961,578226834703,8097153012585,113373983463391,
%U A075921 1587332657497881,22223335428043183,311131443554114505
%N A075921 Second column of triangle A075502.
%C A075921 The e.g.f. given below is Sum_{m=0..1} A075513(2,m)*exp(7*(m+1)*x).
%H A075921 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (21, -98).
%F A075921 a(n) = A075502(n+2, 2) = (7^n)*S2(n+2, 2) with S2(n, m) := A008277(n, m) (Stirling2).
%F A075921 a(n) = -7^n + 2*14^n.
%F A075921 G.f.: 1/((1-7*x)*(1-14*x)).
%F A075921 E.g.f.: (d^2/dx^2)(((exp(7*x)-1)/7)^2)/2! = -exp(7*x) + 2*exp(14*x).
%F A075921 a(0)=1, a(1)=21, a(n) = 21a(n-1) - 98a(n-2). - _Harvey P. Dale_, Apr 30 2011
%t A075921 Table[-7^n+2 14^n,{n,0,20}] (* or *) LinearRecurrence[{21,-98}, {1,21},20] (* _Harvey P. Dale_, Apr 30 2011 *)
%Y A075921 Cf. A000420 (first column), A075922.
%K A075921 nonn,easy
%O A075921 0,2
%A A075921 _Wolfdieter Lang_, Oct 02 2002