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 A024206 Expansion of x^2*(1+x-x^2)/((1-x^2)*(1-x)^2). 39
 0, 1, 3, 5, 8, 11, 15, 19, 24, 29, 35, 41, 48, 55, 63, 71, 80, 89, 99, 109, 120, 131, 143, 155, 168, 181, 195, 209, 224, 239, 255, 271, 288, 305, 323, 341, 360, 379, 399, 419, 440, 461, 483, 505, 528, 551, 575, 599, 624, 649, 675, 701, 728, 755, 783, 811, 840 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(n+1) is the number of 2 X n binary matrices with no zero rows or columns, up to row and column permutation. [ (4th elementary symmetric function of S(n))/(3rd elementary symmetric function of S(n)) ], where S(n) = {first n+3 odd positive integers}. First differences are 1, 2, 2, 3, 3, 4, 4, 5, 5, ... . Let M_n denotes the n X n matrix m(i,j) = 1 if i =j; m(i,j) = 1 if (i+j) is odd; m(i,j) = 0 if i+j is even, then a(n) = -det M_(n+1) - Benoit Cloitre, Jun 19 2002 a(n) is the number of squares with corners on an n X n grid, distinct up to translation. See also A002415, A108279. Starting (1, 3, 5, 8, 11, ...), = row sums of triangle A135841. - Gary W. Adamson, Dec 01 2007 Number of solutions to x+y >= n-1 in integers x,y with 1 <= x <= y <= n-1. - Franz Vrabec, Feb 22 2008 Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=-1, A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=5, a(n-4)=-coeff(charpoly(A,x),x^2). - Milan Janjic, Jan 26 2010 Equals row sums of a triangle with alternate columns of (1,2,3,...) and (1,1,1,...). - Gary W. Adamson, May 21 2010 Conjecture: if a(n) = p#(primorial)-1 for some prime number p, then q=(n+1) is also a prime number where p#=floor(q^2/4). Tested up to n=10^100000 no counterexamples are found. It seems that the subsequence is very scattered. So far the triples (p,q,a(q-1)) are {(2,3,1), (3,5,5), (5,11,29), (7,29,209), (17,1429,510509)}. - David Morales Marciel, Oct 02 2015 Numbers of an Ulam spiral starting at 0 in which the shape of the spiral is exactly a rectangle. E.g. a(4)=5 the Ulam spiral is including at that moment only the elements 0,1,2,3,4,5 and the shape is a rectangle. The area is always a(n)+1. E.g. for a(4) the area of the rectangle is 2(rows)x3(columns)=6=a(4)+1. - David Morales Marciel, Apr 05 2016 LINKS Muniru A Asiru, Table of n, a(n) for n = 1..5000 T. M. Brown, On the unimodality of convolutions of sequences of binomial coefficients, arXiv:1810.08235 [math.CO] (2018). See p. 15. W. F. Lunnon, A postage stamp problem, Comput. J. 12 (1969) 377-380. Thomas Wieder, The number of certain k-combinations of an n-set, Applied Mathematics Electronic Notes, vol. 8 (2008). Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA G.f.: x^2*(1+x-x^2)/((1-x^2)*(1-x)^2). a(n+1) = A002623(n) - A002623(n-1) - 1. a(n) = A002620(n+1)-1. a(n+1) = A002620(n) + n, n>=0. - Philippe Deléham, Feb 27 2004 a(0)=0, a(n) = floor(a(n-1) + sqrt(a(n-1)) + 1) for n > 0. - Gerald McGarvey, Jul 30 2004 a(n) = floor((n+1)^2/4) - 1. - Franz Vrabec, Feb 22 2008 a(n) = A005744(n-1) - A005744(n-2). - R. J. Mathar, Nov 04 2008 a(n) = a(n-1) + [side length of the least square > a(n-1) ], that is a(n)= a(n-1) +  ceiling(sqrt(a(n-1) + 1)). - Ctibor O. Zizka, Oct 06 2009 For a(1)=0, a(2)=1, a(n) = 2*a(n-1) - a(n-2) + 1 if n is odd; a(n) = 2*a(n-1) - a(n-2) if n is even. - Vincenzo Librandi, Dec 23 2010 a(n) = A181971(n, n-1) for n > 0. - Reinhard Zumkeller, Jul 09 2012 a(1)=0, a(2)=1, a(3)=3, a(4)=5, a(n) = 2*a(n-1) - 2*a(n-3) + a(n-4). - Harvey P. Dale, Jun 14 2013 a(n) = floor( (n-1)(n+3)/4 ). - Wesley Ivan Hurt, Jun 23 2013 a(n) = (2*n^2 + 4*n - 7 - (-1)^n)/8. - Wesley Ivan Hurt, Jul 22 2014 a(n) = a(-n-2) = n-1 + floor( (n-1)^2/4 ). [Bruno Berselli, Feb 03 2015] EXAMPLE There are five 2 X 3 binary matrices with no zero rows or columns up to row and column permutation: [1 0 0] [1 0 0] [1 1 0] [1 1 0] [1 1 1] [0 1 1] [1 1 1] [0 1 1] [1 1 1] [1 1 1]. MAPLE A024206:=n->(2*n^2+4*n-7-(-1)^n)/8: seq(A024206(n), n=1..100); MATHEMATICA f[x_, y_] := Floor[ Abs[ y/x - x/y]]; Table[ Floor[ f[2, n^2 + 2 n - 2] /2], {n, 57}] (* Robert G. Wilson v, Aug 11 2010 *) LinearRecurrence[{2, 0, -2, 1}, {0, 1, 3, 5}, 60] (* Harvey P. Dale, Jun 14 2013 *) Rest[CoefficientList[Series[x^2 (1 + x - x^2)/((1 - x^2) (1 - x)^2), {x, 0, 70}], x]] (* Vincenzo Librandi, Oct 02 2015 *) PROG (PARI) a(n)=(n-1)*(n+3)\4 \\ Charles R Greathouse IV, Jun 26 2013 (PARI) x='x+O('x^99); concat(0, Vec(x^2*(1+x-x^2)/ ((1-x^2)*(1-x)^2))) \\ Altug Alkan, Apr 05 2016 (Haskell) a024206 n = (n - 1) * (n + 3) `div` 4 a024206_list = scanl (+) 0 \$ tail a008619_list -- Reinhard Zumkeller, Dec 18 2013 (MAGMA) [(2*n^2+4*n-7-(-1)^n)/8 : n in [1..100]]; // Wesley Ivan Hurt, Jul 22 2014 (GAP) a:=[0, 1, 3, 5];; for n in [5..65] do a[n]:=2*a[n-1]-2*a[n-3]+a[n-4]; od; a; # Muniru A Asiru, Oct 23 2018 CROSSREFS Cf. A135841. Equals A014616 + 1. A row or column of the array A196416 (possibly with 1 subtracted from it). Cf. A008619. Second column of A232206. Sequence in context: A024169 A213706 A078126 * A159325 A228848 A049706 Adjacent sequences:  A024203 A024204 A024205 * A024207 A024208 A024209 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS Corrected and extended by Vladeta Jovovic, Jun 02 2000 STATUS approved

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Last modified May 24 19:14 EDT 2020. Contains 334580 sequences. (Running on oeis4.)