OFFSET
1,3
COMMENTS
That is, the points are chosen from the 2n-1 points on two adjacent sides of the n X n lattice.
LINKS
Eric Weisstein's World of Mathematics, Visible Point
EXAMPLE
a(11)= 4 because all 121 points are visible from (1,1), (1,2), (2,1), and (1,4).
a(25)= 4 because all 625 points are visible from (1,2), (4,1), (6,1), and (23,1).
MATHEMATICA
Join[{1}, Table[hidden=Table[{}, {n^2}]; edgePts={}; Do[pt1=(c-1)*n+d; If[c==1||d==1, 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[2n-1, k]; i=0; While[i<len, i++; s=Subsets[edgePts, {k}, {i}][[1]]; If[Intersection@@hidden[[s]]=={}, done=True; Break[]]]]; k, {n, 2, 11}]]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
T. D. Noe, Mar 06 2009
EXTENSIONS
More terms from Lars Blomberg, Nov 06 2014
STATUS
approved