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 A137932 Terms in an n X n spiral that do not lie on its principal diagonals. 31
 0, 0, 0, 4, 8, 16, 24, 36, 48, 64, 80, 100, 120, 144, 168, 196, 224, 256, 288, 324, 360, 400, 440, 484, 528, 576, 624, 676, 728, 784, 840, 900, 960, 1024, 1088, 1156, 1224, 1296, 1368, 1444, 1520, 1600, 1680, 1764, 1848, 1936, 2024, 2116, 2208, 2304, 2400, 2500, 2600, 2704, 2808 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS The count of terms not on the principal diagonals is always even. The last digit is the repeating pattern 0,0,0,4,8,6,4,6,8,4, which is palindromic if the leading 0's are removed, 4864684. The sum of the last digits is 40, which is the count of the pattern times 4. A 4 X 4 spiral is the only spiral, aside from a 0 X 0, whose count of terms that do not lay on its principal diagonals equal the count of terms that do [A137932(4) = A042948(4)] making the 4 X 4 the "perfect spiral". Yet another property is mod(a(n), A042948(n)) = 0 iff n is even. This is a large family that includes the 4 X 4 spiral. a(n) is the maximum number of queens of one color that can coexist without attacking one queen of the opponent's color on an [n+1] X [n+1] chessboard, when the lone queen is in the most vulnerable position on the board, i.e., on a center square. - Bob Selcoe, Feb 12 2015 Also the circumference of the (n-1) X (n-1) grid graph for n > 2. - Eric W. Weisstein, Mar 25 2018 Also the crossing number of the complete bipartite graph K_{5,n}. - Eric W. Weisstein, Sep 11 2018 LINKS Enrique Pérez Herrero, Table of n, a(n) for n = 0..5000 Kival Ngaokrajang, Illustration of initial terms Eric Weisstein's World of Mathematics, Complete Bipartite Graph Eric Weisstein's World of Mathematics, Graph Circumference Eric Weisstein's World of Mathematics, Graph Crossing Number Eric Weisstein's World of Mathematics, Grid Graph Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA a(n) = n^2 - (2*n - mod(n,2)) = n^2 - A042948(n). a(n) = 2*A007590(n-1). - Enrique Pérez Herrero, Jul 04 2012 G.f. -4*x^3 / ( (1+x)*(x-1)^3 ). a(n) = 4*A002620(n-1). - R. J. Mathar, Jul 06 2012 From Bob Selcoe, Feb 12 2015: (Start) a(n) = (n-1)^2 when n is odd; a(n) = (n-1)^2 - 1 when n is even. a(n) = A002378(n) - A047238(n+1). (End) EXAMPLE a(0) = 0^2 - (2(0) - mod(0,2)) = 0. a(3) = 3^2 - (2(3) - mod(3,2)) = 4. MAPLE A137932:=n->2*floor((n-1)^2/2); seq(A137932(n), n=0..50); # Wesley Ivan Hurt, Apr 19 2014 MATHEMATICA Table[2 Floor[(n - 1)^2/2], {n, 0, 20}] (* Enrique Pérez Herrero, Jul 04 2012 *) 2 Floor[(Range - 1)^2/2] (* Eric W. Weisstein, Sep 11 2018 *) Table[n^2 - 2 n + (1 - (-1)^n)/2, {n, 20}] (* Eric W. Weisstein, Sep 11 2018 *) LinearRecurrence[{2, 0, -2, 1}, {0, 0, 4, 8}, 20] (* Eric W. Weisstein, Sep 11 2018 *) CoefficientList[Series[-((4 x^2)/((-1 + x)^3 (1 + x))), {x, 0, 20}], x] (* Eric W. Weisstein, Sep 11 2018 *) PROG (Python) a = lambda n: n**2 - (2*n - (n%2)) (PARI) A137932(n)={ return(n^2 - (2*n-n%2))} ; print(vector(30, n, A137932(n-1))); /* R. J. Mathar, May 12 2014 */ CROSSREFS Cf. A042948. Sequences on the four axes of the square spiral: Starting at 0: A001107, A033991, A007742, A033954; starting at 1: A054552, A054556, A054567, A033951. Sequences on the four diagonals of the square spiral: Starting at 0: A002939 = 2*A000384, A016742 = 4*A000290, A002943 = 2*A014105, A033996 = 8*A000217; starting at 1: A054554, A053755, A054569, A016754. Sequences obtained by reading alternate terms on the X and Y axes and the two main diagonals of the square spiral: Starting at 0: A035608, A156859, A002378 = 2*A000217, A137932 = 4*A002620; starting at 1: A317186, A267682, A002061, A080335. Sequence in context: A308985 A046059 A290498 * A309141 A329882 A309181 Adjacent sequences:  A137929 A137930 A137931 * A137933 A137934 A137935 KEYWORD nonn,easy AUTHOR William A. Tedeschi, Feb 29 2008 STATUS approved

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Last modified December 1 02:39 EST 2020. Contains 338831 sequences. (Running on oeis4.)