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 A057077 Periodic sequence 1,1,-1,-1; expansion of (1+x)/(1+x^2). 79
 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Sum_{k>=0} a(k)/(k+1) = Sum_{k>=0} 1/((a(k)*(k+1))) = log(2)/2 + Pi/4. - Jaume Oliver Lafont, Apr 30 2010 Abscissa of the image produced after n alternating reflections of (1,1) over the x and y axes respectively.  Similarly, the ordinate of the image produced after n alternating reflections of (1,1) over the y and x axes respectively. - Wesley Ivan Hurt, Jul 06 2013 LINKS Alex Fink, Richard K. Guy, and Mark Krusemeyer, Partitions with parts occurring at most thrice, Contributions to Discrete Mathematics, Vol 3, No 2 (2008), pp. 76-114. See Section 13. T.-X. He, L. W. Shapiro, Fuss-Catalan matrices, their weighted sums, and stabilizer subgroups of the Riordan group, Lin. Alg. Applic. 532 (2017) 25-41, Theorem 2.5, k=2. Index entries for linear recurrences with constant coefficients, signature (0,-1). FORMULA G.f.: (1+x)/(1+x^2). a(n) = S(n, 0) + S(n-1, 0) = S(2*n, sqrt(2)); S(n, x) := U(n, x/2), Chebyshev polynomials of 2nd kind, A049310. S(n, 0)=A056594. a(n) = cos(n*Pi/2) + sin(n*Pi/2) with n >= 0. - Paolo P. Lava, Jun 12 2006 a(n) = (-1)^binomial(n,2) = (-1)^floor(n/2) = 1/2*((n+2) mod 4 - n mod 4). For fixed r = 0,1,2,..., it appears that (-1)^binomial(n,2^r) gives a periodic sequence of period 2^(r+1), the period consisting of a block of 2^r plus ones followed by a block of 2^r minus ones. See A033999 (r = 0), A143621 (r = 2) and A143622 (r = 3). Define E(k) = sum {n = 0..inf} a(n)*n^k/n! for k = 0,1,2,... . Then E(0) = cos(1) + sin(1), E(1) = cos(1) - sin(1) and E(k) is an integral linear combination of E(0) and E(1) (a Dobinski-type relation). Precisely, E(k) = A121867(k) * E(0) - A121868(k) * E(1). See A143623 and A143624 for the decimal expansions of E(0) and E(1) respectively. For a fixed value of r, similar relations hold between the values of the sums E_r(k) := Sum_{n>=0} (-1)^floor(n/r)*n^k/n!, k = 0,1,2,... . For particular cases see A000587 (r = 1) and A143628 (r = 3). - Peter Bala, Aug 28 2008 a(n) = (1/2)*((1-i)*i^n + (1+i)*(-i)^n), with i=sqrt(-1). - Paolo P. Lava, May 26 2010 a(n) = (-1)^A180969(1,n), where the first index in A180969(.,.) is the row index. - Adriano Caroli, Nov 18 2010 a(n) = (-1)^((2*n+(-1)^n-1)/4) = i^((n-1)*n), with i=sqrt(-1). - Bruno Berselli, Dec 27 2010 - Aug 26 2011 Non-simple continued fraction expansion of (3+sqrt(5))/2 = A104457. - R. J. Mathar, Mar 08 2012 E.g.f.: cos(x)*(1 + tan(x)). - Arkadiusz Wesolowski, Aug 31 2012 From Ricardo Soares Vieira, Oct 15 2019: (Start) E.g.f.: sin(x) + cos(x) = sqrt(2)*sin(x + Pi/4). a(n) = sqrt(2)*(d^n/dx^n) sin(x)|_x=Pi/4, i.e., a(n) equals sqrt(2) times the n-th derivative of sin(x) evaluated at x=Pi/4. (End) MAPLE seq((-1)^floor(k/2), k=0..70); # Wesley Ivan Hurt, Jul 06 2013 MATHEMATICA a[n_] := {1, 1, -1, -1}[[Mod[n, 4] + 1]] (* Jean-François Alcover, Jul 05 2013 *) PadRight[{}, 80, {1, 1, -1, -1}] (* Harvey P. Dale, Jun 21 2015 *) PROG (Maxima) A057077(n) := block(         [1, 1, -1, -1][1+mod(n, 4)] )\$ /* R. J. Mathar, Mar 19 2012 */ (MAGMA) &cat[[1, 1, -1, -1]^^20]; // Vincenzo Librandi, Feb 18 2016 (PARI) a(n)=(-1)^(n\2) \\ Charles R Greathouse IV, Nov 07 2016 CROSSREFS |a(n)|=A000012. Cf. A049310. Cf. A000587, A121867, A121868, A130151, A143621, A143622, A143623, A143624, A143628. Sequence in context: A153881 A160357 A186039 * A262725 A070748 A154990 Adjacent sequences:  A057074 A057075 A057076 * A057078 A057079 A057080 KEYWORD sign,easy AUTHOR Wolfdieter Lang, Aug 04 2000 STATUS approved

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Last modified December 6 14:15 EST 2019. Contains 329806 sequences. (Running on oeis4.)