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 A034935 #19 Aug 01 2017 09:50:45
%S A034935 0,2,7,57,182,2057,14557,45807,280182,6139557,25670807,123327057,
%T A034935 5006139557,11109655182,102662389557,407838170807,3459595983307,
%U A034935 79753541295807,365855836217682,2273204469030182,49956920289342682
%N A034935 Successive approximations to 5-adic integer sqrt(-1).
%C A034935 This is the root congruent to 2 mod 5.
%D A034935 J. H. Conway, The Sensual Quadratic Form, p. 118.
%D A034935 K. Mahler, Introduction to p-Adic Numbers and Their Functions, Cambridge, 1973, p. 35.
%H A034935 Seiichi Manyama, <a href="/A034935/b034935.txt">Table of n, a(n) for n = 0..1121</a>
%F A034935 Successive values of 2^(5^x) mod 5^x. - Joe K. Crump (joecr(AT)carolina.rr.com), Jan 20 2001
%t A034935 Table[ PowerMod[2, 5^n, 5^n], {n, 0, 24}] // Union (* _Jean-François Alcover_, Dec 03 2012, from formula given by Joe K. Crump *)
%o A034935 (PARI) sqrt(-1+O(5^40))
%o A034935 (PARI) {a(n) = local(k, x, y); for(i = 0, n, until( x != (y = truncate( sqrt( -1 + O(5^(k++))))), x = y));x} /* _Michael Somos_, Mar 03 2008 */
%Y A034935 Cf. A034939, A048898, A048899.
%K A034935 nonn,easy,nice
%O A034935 0,2
%A A034935 _N. J. A. Sloane_