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 A138998 #6 Feb 18 2017 16:34:46
%S A138998 0,1,1,1,1,9,49,121,65,329,913,25,2273,8169,16177,30905,16001,12937,
%T A138998 116433,523609,1042465,2042153,1602161,6030841,12334529,10347465,
%U A138998 26018321,32838297,27109217,512418409,316798385,703701817,2038349057,1165272329,10487450961
%N A138998 9^n mod 2^n.
%H A138998 G. C. Greubel, <a href="/A138998/b138998.txt">Table of n, a(n) for n = 0..1000</a>
%t A138998 Table[Mod[9^n, 2^n], {n,0,50}] (* _G. C. Greubel_, Feb 18 2017 *)
%K A138998 nonn
%O A138998 0,6
%A A138998 _N. J. A. Sloane_, May 20 2008