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 A258643 Irregular triangle read by rows, n >= 1, k >= 0: T(n,k) is the number of distinct patterns of n X n squares with k holes that are squares (see the construction rule in comments). 4
 1, 1, 2, 1, 3, 1, 2, 9, 7, 4, 4, 5, 2, 25, 11, 40, 8, 33, 3, 16, 0, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The sequence of row lengths is A261243. - Wolfdieter Lang, Aug 18 2015 The construction rules are: (o) The n X n square has horizontal and vertical diagonals. (i) A pattern must be symmetric with respect to both vertical and horizontal axes. (ii) For n >= 2, each pattern must have four squares at the corners. (iii) The squares must have continuity contact to each other either by sides or corners. (iv) The hole(s) must be square(s). Mirror patterns with respect to the main diagonal are not considered as different. See illustration in the links. Each pattern can be a seed of a box fractal; e.g., the second pattern of T(3,0), consisting of 5 squares and 0 holes, is a seed of the Vicsek fractal (see a link below); the second pattern of T(4,2), consisting of 10 squares and 2 holes, is a seed of the fractal in a link of A002276. If the figures are rotated by 45 degrees in the clockwise direction they can be considered as binary bisymmetric n X n matrices B_n if a red square stand for 1 and an empty square for 0. The four corners have entries 1, that is B_n[1, 1] = 1 = B_n[1, n]. The continuity of the red squares, mentioned above in point (iii), means that there is no rectangular path of 0's (no diagonal steps) in the matrix B_n that dissects it into two parts. See A261242 for more details, where also the figures with nonsquare holes and the mirrors (row reversion in the B_n matrix) are considered. - Wolfdieter Lang, Aug 18 2015 LINKS Kival Ngaokrajang, Illustration of pattern T(n,k) for n = 1..5, k >= 0, T(6,k) patterns Wikipedia, Vicsek fractal EXAMPLE Irregular triangle begins: n\k  0   1   2  3   4  5   6  7  8 ... 1    1 2    1 3    2   1 4    3   1   2 5    9   7   4  4   5  2 6   25  11  40  8  33  3  16  0  4 ... CROSSREFS Cf. A002276 (10 squares, 2 holes), A016203 (8 squares, 0 holes), A023001 (8 squares, 1 hole), A218724 (21 squares, 4 holes). Cf. A261242, A261243. Sequence in context: A168017 A293980 A240694 * A287561 A046924 A015710 Adjacent sequences:  A258640 A258641 A258642 * A258644 A258645 A258646 KEYWORD nonn,tabf,more AUTHOR Kival Ngaokrajang, Jun 06 2015 STATUS approved

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Last modified September 24 07:01 EDT 2020. Contains 337317 sequences. (Running on oeis4.)