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 A261242 Irregular triangle T(n, k) of number of connected bisymmetric n X n matrices B_n with 0 or 1 entries, B_n[1,1] = 1  = B_n[1,n], and k islands of 0s. 5
 1, 1, 2, 1, 4, 1, 4, 12, 18, 12, 8, 6, 2, 44, 56, 120, 28, 88, 4, 36, 0, 8 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The row length sequence is 1 for n = 1 and A000982(n-2) + 1 for n >= 2, that is:  1, 1, 2, 3, 6, 9, 14, 19, 26, 33, 42, ... = A261243. This entry is motivated by A258643. For bisymmetric matrices see the Wikipedia link. For the number of independent entries of an n X n bisymmetric matrix B_n see a Jul 07 2015 comment on A002620(n+1), n >= 1. For the binary case (only 0 and 1 entries) see A060656(n+1), and the Dennis P. Walsh comment and link. If B_n[1,1] and B_n[1,n] is given then the four corners are fixed, and, for n >= 3, there are A002620(n+1) - 2 = A014616(n-2) entries free. If the n X n bisymmetric matrix B_n of 0s and 1s with B_n[1, 1] = 1 = B_n[1, n] is considered as a grid of n^2 squares of length 1 (in some length unit) with the four corners filled with 1s and the other squares with 0 or 1 then a path between the centers of squares with step length 1 can be defined. No diagonal steps (length sqrt(2)) are allowed. B_n is called connected if there exists no path of 0s which dissects the grid into two parts. An island of 0s (a 0-island) in B_n is defined as a set of 0s for which each pair is connected by a path of 0s, and a 0 entry at the coast of a 0-island has at least one entry 1 one step away. A single square filled with a 0 is a 0-island if all four neighbors 1 step (of length 1)apart are filled with 1s. If k=0 there exists no such 0-island. See the n=4 examples with k >=1 below. The k = 1 matrix has one simply connected 0-island of four squares. The four k = 2 matrices have two 0-islands consisting of one square each. See the link with the figures by K. N. where red squares stand for 1 and empty squares for 0. Each matrix appears there rotated by 45 degrees in the counterclockwise direction. The mirror operation means row reversion in the matrix B_n. In the figures this is a mirror operation w.r.t. the middle NW-SE diagonal. 0-islands appear in the figures as holes. For the row sums see A261244. LINKS Kival Ngaokrajang, Illustration of T(n,k) for n = 1..5, k >= 0, T(6,0), T(6,1), T(6,2), T(6,4), T(6,k) for k = 3, 5, 6, 8 Wikipedia, Bisymmetric Matrix. EXAMPLE The irregular triangle T(n, k) begins: n\k   0   1    2   3   4  5   6   7   8  ... 1:    1 2:    1 3:    2   1 4:    4   1    4 5:   12  18   12   8   6  2 6:   44  56  120  28  88  4  36   0   8 ... n=4: k=0: [[1,1,1,1], [1,1,1,1], [1,1,1,1], [1,1,1,1]], [[1,0,0,1], [0,1,1,0], [0,1,1,0], [1,0,0,1]], [[1,1,0,1], [1,1,1,0], [0,1,1,1], [1,0,1,1]], [[1,0,1,1], [0,1,1,1], [1,1,1,0], [1,1,0,1]];      k=1: [[1,1,1,1], [1,0,0,1], [1,0,0,1], [1,1,1,1]];      k=2: [[1,1,1,1], [1,0,1,1], [1,1,0,1], [1,1,1,1]], [[1,1,1,1], [1,1,0,1], [1,0,1,1], [1,1,1,1]], [[1,1,0,1], [1,0,1,0], [0,1,0,1], [1,0,1,1]], [[1,0,1,1], [0,1,0,1], [1,0,1,0], [1,1,0,1]]. CROSSREFS Cf. A000982, A002620, A014616, A258643, A261243, A261244. Sequence in context: A102627 A296560 A284652 * A088296 A282738 A093890 Adjacent sequences:  A261239 A261240 A261241 * A261243 A261244 A261245 KEYWORD nonn,tabf,more AUTHOR Wolfdieter Lang and Kival Ngaokrajang, Aug 18 2015 STATUS approved

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Last modified February 26 00:08 EST 2020. Contains 332270 sequences. (Running on oeis4.)