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 A325840 #5 Jun 09 2019 13:09:28
%S A325840 1,1,-2,4,-8,15,-25,35,-35,0,126,-462,1253,-2989,6615,-13867,27734,
%T A325840 -52921,95863,-163115,255493,-352716,379050,-134596,-828875,3407821,
%U A325840 -9217717,20837509,-41675018,74480265,-116145145,143502875,-78990635,-286097760,1453179125
%N A325840 First term of n-th difference sequence of (round(k*sqrt(2))), k >= 0.
%H A325840 Clark Kimberling, <a href="/A325840/b325840.txt">Table of n, a(n) for n = 1..200</a>
%t A325840 Table[First[Differences[Table[Round[Sqrt[2]*n], {n, 0, 50}], n]], {n, 1, 50}]
%Y A325840 Cf. A325664.
%K A325840 easy,sign
%O A325840 1,3
%A A325840 _Clark Kimberling_, Jun 09 2019