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 A023961 #35 Oct 28 2019 11:27:04
%S A023961 0,4,7,0,2,4,6,8,0,1,3,4,6,7,8,0,1,2,3,4,5,6,7,8,0,0,1,2,3,4,5,6,7,8,
%T A023961 9,0,0,1,2,3,4,4,5,6,7,7,8,9,0,0,1,2,2,3,4,4,5,6,6,7,8,8,9,0,0,1,1,2,
%U A023961 3,3,4,4,5,6,6,7,7,8,8,9,0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,0,0,0,1,1,2
%N A023961 First digit after decimal point of square root of n.
%C A023961 When n is a square, a(n) is equal to 0, but the converse is not true, see A034096. - _Michel Marcus_, Sep 21 2015
%H A023961 Nathaniel Johnston, <a href="/A023961/b023961.txt">Table of n, a(n) for n = 1..10000</a>
%F A023961 a(n) = A010879(A000196(100*n)). - _Robert Israel_, Jul 30 2015
%e A023961 sqrt(1) = 1.00000000... hence a(1) = 0.
%e A023961 sqrt(2) = 1.41421356... hence a(2) = 4.
%e A023961 sqrt(3) = 1.73205080... hence a(3) = 7.
%e A023961 sqrt(4) = 2.00000000... hence a(4) = 0.
%e A023961 Note that 26 = 2 * 13 and sqrt(26) = 5.09901951... so a(26) = 0 even though 26 is not a perfect square.
%p A023961 A023961 := proc(n) return floor(10*sqrt(n)) mod 10: end: seq(A023961(n),n=1..100); # _Nathaniel Johnston_, May 04 2011
%t A023961 Array[Function[n, RealDigits[N[Power[n, 1/2], 10], 10] // (#[[1, #[[2]] + 1]]) &], 110]
%t A023961 fd[n_] := Module[{rd = RealDigits[Sqrt[n], 10, 10]}, First[rd][[Last[rd] + 1]]]; Array[fd, 90] (* _Harvey P. Dale_, Jan 16 2014 *)
%o A023961 (PARI) a(n) = floor(10*sqrt(n)) % 10; \\ _Michel Marcus_, Sep 21 2015
%Y A023961 Cf. A111862, A111850, A111851, A111852, A111853, A111854, A111855, A111856, A111857, A111858, A111859.
%K A023961 nonn,easy,base
%O A023961 1,2
%A A023961 _N. J. A. Sloane_, _Olivier Gérard_