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 A101946 a(n) = 6*2^n - 3*n - 5. 9
 1, 4, 13, 34, 79, 172, 361, 742, 1507, 3040, 6109, 12250, 24535, 49108, 98257, 196558, 393163, 786376, 1572805, 3145666, 6291391, 12582844, 25165753, 50331574, 100663219, 201326512, 402653101, 805306282, 1610612647, 3221225380, 6442450849, 12884901790 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Sequence generated from a 3 X 3 matrix, companion to A101945. Characteristic polynomial of M = x^3 - 4x^2 + 5x - 2. Sequence can also be generated by the same method as A061777 with slightly different rules. Refer to A061777, which is the "vertex to vertex" expansion version. For this case, the expandable vertices of the existing generation will contact the sides of the new ones, i.e., "vertex to side" expansion version. Let us assign the label "1" to the triangle at the origin; at n-th generation add a triangle at each expandable vertex, i.e., each vertex where the added generations will not overlap the existing ones, although overlaps among new generations are allowed. The non-overlapping triangles will have the same label value as a predecessor; for the overlapping ones, the label value will be sum of label values of predecessors. a(n) is the sum of all label values at n-th generation. The triangles count is A005448. See illustration. - Kival Ngaokrajang, Sep 26 2014 The number of ways to select 0 or more nodes of a 2 X n rectangular grid such that the selected nodes are connected, but do not fill any 2 X 2 square. This question arises in logic puzzles such as Nurikabe. - Hugo van der Sanden, Feb 22 2024 LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 Kival Ngaokrajang, Illustration of initial terms Index entries for linear recurrences with constant coefficients, signature (4,-5,2). FORMULA a(0)=1, a(1)=4, a(2)=13 and for n>2, a(n) = 4*a(n-1) - 5*a(n-2) + 2*a(n-3). a(n) = right term in M^n * [1 1 1], where M = the 3X3 matrix [1 0 0 / 2 2 0 / 1 2 1]. M^n * [1 1 1] = [1 A033484(n) a(n)]. a(0) = 1, for n >= 1, a(n) = 3*A000225(n) + a(n-1). - Kival Ngaokrajang, Sep 26 2014 G.f.: (1+2*x^2)/((1-x)^2*(1-2*x)). - Colin Barker, Sep 26 2014 E.g.f.: 6*exp(2*x) - (5+3*x)*exp(x). - G. C. Greubel, Feb 06 2022 EXAMPLE a(4) = 79 = 4*34 - 5*13 + 2*4 = 4*a(3) - 5*a(2) + 2*a(1). a(4) = right term in M^4 * [1 1 1], since M^4 * [1 1 1] = [1 46 a(4)], where 46 = A033484(4). MATHEMATICA a[0]=1; a[1]=4; a[2]=13; a[n_]:= a[n]= 4a[n-1] -5a[n-2] +2a[n-3]; Table[ a[n], {n, 0, 30}] (* Or *) a[n_] := (MatrixPower[{{1, 0, 0}, {2, 2, 0}, {1, 2, 1}}, n].{{1}, {1}, {1}})[[3, 1]]; Table[ a[n], {n, 0, 30}] (* Robert G. Wilson v, Jan 12 2005 *) Table[6*2^n-3n-5, {n, 0, 40}] (* or *) LinearRecurrence[{4, -5, 2}, {1, 4, 13}, 40] (* Harvey P. Dale, Jun 03 2017 *) PROG (PARI) a(n) = if (n<1, 1, 5*(2^n-1)+a(n-1))\\ Kival Ngaokrajang, Sep 26 2014 (PARI) Vec(-(2*x^2+1)/((x-1)^2*(2*x-1)) + O(x^100)) \\ Colin Barker, Sep 26 2014 (Magma) [6*2^n -3*n-5: n in [0..40]]; // G. C. Greubel, Feb 06 2022 (Sage) [3*(2^(n+1) -n-2) +1 for n in (0..40)] # G. C. Greubel, Feb 06 2022 CROSSREFS Cf. A033484, A101945. Cf. A000225, A005448, A061777. Sequence in context: A322599 A135859 A161531 * A029860 A262200 A213578 Adjacent sequences: A101943 A101944 A101945 * A101947 A101948 A101949 KEYWORD nonn,easy AUTHOR Gary W. Adamson, Dec 22 2004 EXTENSIONS New definition from Ralf Stephan, May 17 2007 STATUS approved

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Last modified April 25 09:31 EDT 2024. Contains 371967 sequences. (Running on oeis4.)