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 A239072 Maximum number of cells in a square n X n grid that can be painted such that no two orthogonally adjacent cells are painted, and that every unpainted cell can be reached from the edge of the grid by a series of orthogonal steps to unpainted cells. 2
 0, 1, 2, 5, 7, 11, 15, 21, 26, 32, 39, 47, 54, 64, 74, 85, 94, 107, 119, 132, 146, 160, 174, 191, 206, 223 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS This sequence has implications for constructing Steiner trees for square unit arrays of dots: an orthogonal-only tree for an m X m dot array would need m^2-1 units. The value of a(m-1) tells you how many (1+sqrt(3)) 'X' shapes you can place in the grid, each saving (2-sqrt(3)) units. Unfortunately this doesn't generally lead to the minimal Steiner tree. An upper bound for this sequence is (((n+1)^2)-1)/3, which is reached iff n = 2^k-1. The value of a(n)/n^2 (the painted cells as a proportion of all of the cells) converges extremely slowly to 1/3. This sequence is related to the sequence of Heyawake numbers A239231, which has the additional criterion that the unpainted area should be contiguous. For sufficiently large Heyawake grids, this sequence forms the central n-4 X n-4 square of the Heyawake grid. LINKS EXAMPLE For example, if n=6, the painted cells could be A1, A3, A5, B2, B6, C1, C3, C5, D6, E1, E3, E5, F2, F4, F6 (15 cells in all). CROSSREFS Cf. A239231, a similar sequence, with one extra criterion - that the unpainted cells should be contiguous. Sequence in context: A032616 A006066 A084935 * A317242 A217302 A062409 Adjacent sequences:  A239069 A239070 A239071 * A239073 A239074 A239075 KEYWORD nonn,more AUTHOR Elliott Line, Mar 10 2014 EXTENSIONS Some values corrected, incorrect formula and values removed by Elliott Line, Aug 21 2014 STATUS approved

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Last modified October 22 01:41 EDT 2018. Contains 316431 sequences. (Running on oeis4.)