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 A176742 Expansion of (1 - x^2) / (1 + x^2) in powers of x. 7
 1, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2, 0, -2, 0, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Difference sequence of A057077. Sequence of determinants of matrices for some bipartite graphs, called Tz(n). The graph Tz(4) appears in the logo for the beer called Tannenzaepfle (small fir cone), brewed by Badische Staatsbrauerei Rothaus, Germany, hence the name Tz. See the link for this logo with Tz(4). The vertex-vertex matrix for these bipartite graphs will also be called Tz(n) (without leading to confusion). General proof by expanding the determinant a(n) = determinant(Tz(n)) along the first column yielding b(n-1)-b(n-2), with b(n-1) the A_{1,1} minor of the matrix Tz(n), and deriving a recurrence for the b(n), namely b(n) = -b(n-2) with inputs b(0) = 1 = b(1). This gives b(n) = A057077(n), n>=0. LINKS W. Lang, Some Tz(n) graphs and matrices. [From Wolfdieter Lang, Oct 28 2010] Index entries for linear recurrences with constant coefficients, signature (0,-1) FORMULA Euler transform of length 4 sequence [ 0, -2, 0, 1]. - Michael Somos, Mar 21 2011 Moebius transform is length 4 sequence [ 0, -2, 0, 4]. - Michael Somos, Mar 22 2011 a(n) = a(-n) for all n in Z. a(n) = c_4(n) if n>1, where c_k(n) is Ramanujan's sum. - Michael Somos, Mar 21 2011 a(n-1) := determinant(Tz(n)), n>=1. The rows of the matrix Tz(4) are [[1, 1, 0, 0], [1, 0, 1, 0], [0, 1, 0, 1], [0, 0, 1, 1]]. Tz(1)=(1), and Tz(2) has rows [[1, 1], [1, 1]]. The matrix for the generalization Tz(n) has rows [[1,1,0,...,0], [1,0,1,0,...,0], [0,1,0,1,0,...,0], ..., [0,...,0,1,0,1], [0,...,0,1,1]. a(0)=1, a(2*k-1)= 0, a(4*k) = +2, a(4*k-2) = -2, k>=1. O.g.f.: (1-x^2)/(1+x^2). a(n) = A057077(n) - A057077(n-1), n>=1. a(0)=1. Dirichlet g.f. sum_{n>=1} a(n)/n^s = zeta(s)*(4^(1-s)-2^(1-s)). - R. J. Mathar, Apr 11 2011 a(n) = (((n+1) mod 2)+((n+2+sign(n)) mod 2))*(-1)^ceiling(n/2). - Wesley Ivan Hurt, Jun 20 2014 EXAMPLE G.f. = 1 - 2*x^2 + 2*x^4 - 2*x^6 + 2*x^8 - 2*x^10 + 2*x^12 - 2*x^14 + 2*x^16 + ... The bipartite graphs Tz(n) (n>=1) look like |, |X|, |XX|, |XXX|, ... For n>=2 the lines have to be connected to give the 2*n nodes and 2*n edges. The n=1 graph Tz(1) has 2*1=2 nodes and only one edge. n=1:determinant((1))=1, n=2: determinant(Matrix([[1,1],[1,1]))=0; n=3: determinant(Matrix([[1,1,0],[1,0,1],[0,1,1]))=-2; n=4: determinant(Tz(4))=0; etc. MAPLE a := n -> 2^signum(n)*(-1)^iquo(n+1, 2)*modp(n+1, 2); seq(a(n), n=0..100); # Peter Luschny, Jun 22 2014 MATHEMATICA Join[{1}, Table[{0, -2, 0, 2}, {26}]] // Flatten (* Jean-François Alcover, Jun 21 2013 *) a[ n_] := -Boole[n == 0] + {0, -2, 0, 2}[[Mod[ n, 4, 1]]]; (* Michael Somos, May 05 2015 *) PROG (PARI) {a(n) = - (n == 0) + [ 2, 0, -2, 0][n%4 + 1]}; /* Michael Somos, Mar 21 2011 */ CROSSREFS Cf. A084099, A057077. Sequence in context: A230103 A267602 A021499 * A010673 A084099 A036665 Adjacent sequences:  A176739 A176740 A176741 * A176743 A176744 A176745 KEYWORD sign,easy AUTHOR Wolfdieter Lang, Oct 15 2010 STATUS approved

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