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 A157720 #6 Nov 06 2014 10:14:43
%S A157720 1,1,2,2,3,3,3,3,3,3,4,3,4,3,4,4,4,4,4,4,4,4,4,4,4,4,5,4,5,4,5,4,5,4,
%T A157720 5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,5,4,
%U A157720 5,4,5,4,5,4,5,4,5,5,5,4,5,5,5,4,5,5,5
%N A157720 Least number of edge lattice points from which every point of a square n x n lattice is visible.
%C A157720 This sequence, which is easier to compute than A157639, provides an upper bound for A157639. By using every other point on one side of the lattice, it is easy to see that a(n) <= ceiling(n/2).
%e A157720 a(3) = 2 because all 9 points are visible from (1,1) or (1,2).
%e A157720 a(5) = 3 because all 25 points are visible from (1,1), (1,2), or (1,4).
%e A157720 a(11)= 4 because all 121 points are visible from (1,1), (1,2), (2,1), or (1,4).
%e A157720 a(27)= 5 because all 729 points are visible from (1,1), (1,2), (2,1), (1,3), or (1,4).
%t A157720 Join[{1}, Table[hidden=Table[{},{n^2}]; edgePts={}; Do[pt1=(c-1)*n+d; If[c==1||c==n||d==1||d==n, AppendTo[edgePts,pt1]; lst={}; Do[pt2=(a-1)*n+b; If[GCD[c-a,d-b]>1, AppendTo[lst,pt2]], {a,n}, {b,n}]; hidden[[pt1]]=lst], {c,n}, {d,n}]; edgePts=Sort[edgePts]; done=False; k=0; done=False; k=0; While[ !done, k++; len=Binomial[4n-4,k]; i=0; While[i<len, i++; s=Subsets[edgePts,{k},{i}][[1]]; If[Intersection@@hidden[[s]]=={}, done=True; Break[]]]]; k, {n,2,11}]]
%K A157720 hard,nonn
%O A157720 1,3
%A A157720 _T. D. Noe_, Mar 06 2009
%E A157720 More terms from _Lars Blomberg_, Nov 06 2014