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 A138589 #22 Oct 04 2017 02:09:19
%S A138589 0,1,9,61,113,53,3337,12589,62945,118117,328441,2690781,9259601,
%T A138589 12743573,197935593,452807053,2264035265,7025209029,35126045145,
%U A138589 106910748989,809431651889,1848135003893,9240675019465,28611189052909,213424689442209,785648470500389
%N A138589 a(n) = 5^n mod 4^n.
%H A138589 Seiichi Manyama, <a href="/A138589/b138589.txt">Table of n, a(n) for n = 0..1661</a>
%t A138589 Table[PowerMod[5, n, 4^n], {n,0,50}] (* _G. C. Greubel_, Oct 01 2017 *)
%o A138589 (PARI) concat([0], vector(50, n, lift(Mod(5, 4^n)^n))) \\ _Michel Marcus_, Oct 02 2017
%Y A138589 Cf. k^n mod (k-1)^n: A002380 (k=3), A064629 (k=4), this sequence (k=5), A138649 (k=6), A139786 (k=7), A138973 (k=8), A139733 (k=9).
%K A138589 nonn
%O A138589 0,3
%A A138589 _N. J. A. Sloane_, May 20 2008