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 A114043 Take an n X n square grid of points in the plane; a(n) = number of ways to divide the points into two sets using a straight line. 13
 1, 7, 29, 87, 201, 419, 749, 1283, 2041, 3107, 4493, 6395, 8745, 11823, 15557, 20075, 25457, 32087, 39725, 48935, 59457, 71555, 85253, 101251, 119041, 139351, 161933, 187255, 215137, 246691, 280917, 319347, 361329, 407303 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, half of the number of two-dimensional threshold functions (A114146). The line may not pass through any point. This is the "labeled" version - rotations and reflections are not taken into account (cf. A116696). The number of ways to divide a (2n) X (2n) grid into two sets of equal size is given by 2*A099957(n). - David Applegate, Feb 23 2006 All terms are odd: the line that misses the grid contributes 1 to the total and all other lines contribute 2, 4 or 8, so the total must be odd. What can be said about the 3-D generalization? - Max Alekseyev, Feb 27 2006 LINKS T. D. Noe, Table of n, a(n) for n=1..1000 Max A. Alekseyev. On the number of two-dimensional threshold functions. SIAM J. Disc. Math. 24(4), 2010, pp. 1617-1631. doi:10.1137/090750184 FORMULA Let V(m,n) = Sum_{i=1..m, j=1..n, gcd(i,j)=1} (m+1-i)*(n+1-j); then a(n+1) = 2*(n^2 + n + V(n,n)) + 1. - Max Alekseyev, Feb 22 2006 a(n) ~ (3/pi^2) * n^4. - Max Alekseyev, Feb 22 2006 a(n) = A141255(n) + 1. - T. D. Noe, Jun 17 2008 EXAMPLE Examples: the two sets are indicated by X's and o's. a(2) = 7: XX oX Xo XX XX oo oX XX XX XX Xo oX XX oX -------------------- a(3) = 29: XXX oXX ooX ooo ooX ooo XXX XXX XXX XXX oXX oXX XXX XXX XXX XXX XXX XXX -1- -4- -8- -4- -4- -8- Total = 29 -------------------- a(4)= 87: XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXo XXXo XXXo XXoo XXoo XXXX XXXo XXoo Xooo oooo XXoo Xooo oooo Xooo oooo --1- --4- --8- --8- --4- --4- --8- --8- --8- --8- XXXX XXXX XXXX XXXX XXXX XXXo XXXX XXXX XXXo XXXo XXoo Xooo oooo Xooo XXoo Xooo oooo oooo oooo oooo --4- --8- --2- --4- --8- Total = 87. -------------------- MATHEMATICA a[n_] := 2*Sum[(n - i)*(n - j)*Boole[CoprimeQ[i, j]], {i, 1, n - 1}, {j, 1, n - 1}] + 2*n^2 - 2*n + 1; Array[a, 40] (* Jean-François Alcover, Apr 25 2016, after Max Alekseyev *) CROSSREFS Cf. A114499, A115004, A115005, A116696 (unlabeled case), A114531, A114146. Cf. A099957. Sequence in context: A079796 A242727 A229795 * A166189 A001779 A257201 Adjacent sequences:  A114040 A114041 A114042 * A114044 A114045 A114046 KEYWORD nonn,nice AUTHOR Ugo Merlone (merlone(AT)econ.unito.it) and N. J. A. Sloane, Feb 22 2006 EXTENSIONS More terms from Max Alekseyev, Feb 22 2006 STATUS approved

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Last modified December 15 00:30 EST 2019. Contains 329988 sequences. (Running on oeis4.)