This site is supported by donations to The OEIS Foundation.

"Email this user" was broken Aug 14 to 9am Aug 16. If you sent someone a message in this period, please send it again.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A056594 Periodic sequence 1,0,-1,0...; expansion of 1/(1+x^2). 82
 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0, -1, 0, 1, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS G.f. is inverse of cyclotomic(4,x). Unsigned: A000035(n+1). Real part of i^n and imaginary part of i^(n+1), i=sqrt(-1). - Reinhard Zumkeller, Jul 22 2007 The BINOMIAL transform generates A009116(n); the inverse BINOMIAL transform generates (-1)^n*A009116(n). - R. J. Mathar, Apr 07 2008 a(n-1), n >= 1, is the nontrivial Dirichlet character modulo 4, called Chi_2(4;n) (the trivial one is Chi_1(4;n) given by periodic(1,0) = A000035(n)). See the Apostol reference, p. 139, the k = 4, phi(k) = 2 table. - Wolfdieter Lang, Jun 21 2011 a(n-1), n >= 1, is the character of the Dirichlet beta function. - Daniel Forgues, Sep 15 2012 a(n-1), n >= 1, is also the (strongly) multiplicative function h(n) of Theorem 5.12, p. 150, of the Niven-Zuckerman reference. See the formula section. This function h(n) can be employed to count the integer solutions to n = x^2 + y^2. See A002654 for a comment with the formula. - Wolfdieter Lang, Apr 19 2013 This sequence is duplicated in A101455 but with offset 1. - Gary Detlefs, Oct 04 2013 REFERENCES T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1986. I. S. Gradstein and I. M. Ryshik, Tables of series, products, and integrals, Volume 1, Verlag Harri Deutsch, 1981. Ivan Niven and Herbert S. Zuckerman, An Introduction to the Theory of Numbers, New York: John Wiley (1980), p. 150. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Eric Weisstein's World of Mathematics, Kronecker Symbol. Wikipedia, Dirichlet beta function Wikipedia, Kronecker Symbol. Index entries for linear recurrences with constant coefficients, signature (0, -1). FORMULA G.f.: 1/(1+x^2). E.g.f.: cos(x). a(n) = (1/2)*((-i)^n + i^n), where i = sqrt(-1). - Mitch Harris, Apr 19 2005 a(n) = (1/2)*((-1)^(n+floor(n/2)) + (-1)^floor(n/2)). Recurrence: a(n)=a(n-4), a(0)=1, a(1)=0, a(2)=-1, a(3)=0. Also a(n) = -a(n-2) for n>1; a(n) = A010892(A001651(n+1)); a(n) = (-(n mod 4)-(n+1 mod 4)+(n+2 mod 4)+(n+3 mod 4))/4 (cf. forms of modular arithmetic of Paolo P. Lava, see A146094). [Bruno Berselli, Feb 08 2011] a(n) = cos(n*Pi/2), with n>=0. - Paolo P. Lava, Aug 02 2006 a(n) = T(n, 0) = A053120(n, 0); T(n, x) Chebyshev polynomials of the first kind. - Wolfdieter Lang, Aug 21 2009 a(n) = S(n, 0)= A049310(n, 0); S(n, x) := U(n, x/2), Chebyshev polynomials of 2nd kind. Sum_{k>=0} a(k)/(k+1) = Pi/4. - Jaume Oliver Lafont, Mar 30 2010 a(n) = Sum_{k, 0<=k<=n} A101950(n,k)*(-1)^k. - Philippe Deléham, Feb 10 2012 a(n) = (1/2)*(1+(-1)^n)*(-1)^(n/2). [Bruno Berselli, Mar 13 2012] a(0) = 1, a(n-1) = 0 if n is even, a(n-1) = product((-1)^(e_j*(p_j-1)/2), j = 1..m) if the odd n-1 = p_1^(e_1)*p_2^(e_2)* ...*p_m^(e_m) with distinct odd primes p_j, j=1..m.  See the function h(n) of Theorem 5.12 of the Niven-Zuckerman reference. - Wolfdieter Lang, Apr 19 2013 a(n) = (-4/(n+1)), n >= 0, where (k/n) is the Kronecker symbol. See the Eric Weisstein and Wikipedia links. Thanks to Wesley Ivan Hurt. - Wolfdieter Lang, May 31 2013 a(n) = R(n,0)/2 with the row polynomials R of A127672. This follows from the product of the zeros of R, and the formula product(2*cos((2*k+1)*Pi/(2*n)), k=0..n-1) = (1 + (-1)^n)*(-1)^(n/2), n >= 1 (see the Gradstein and Ryshik reference, p. 63, 1.396 4., with x = sqrt(-1)). - Wolfdieter Lang, Oct 21 2013 a(n) = Sum_{k=0..n} i^(k*(k+1)), where i=sqrt(-1). [Bruno Berselli, Mar 11 2015] Dirichlet g.f. of a(n) shifted right: L(chi_2(4),s) = beta(s) = (1-2^(-s))*(D.g.f. of A034947), see comments by Lang and Forgues. - Ralf Stephan, Mar 27 2015 EXAMPLE With a(n-1) = h(n) of Niven-Zuckerman: a(62) = h(63) = h(3^2*7^1) = (-1)^(2*1)*(-1)^(1*3) = -1 = h(3)^2*h(7) = a(2)^2*a(6) = (-1)^2*(-1) = -1. - Wolfdieter Lang, Apr 19 2013 MAPLE A056594 := n->(1-irem(n, 2))*(-1)^iquo(n, 2); # Peter Luschny, Jul 27 2011 MATHEMATICA CoefficientList[Series[1/(1 + x^2), {x, 0, 50}], x] a[n_]:= KroneckerSymbol[-4, n+1]; Table[a[n], {n, 0, 93}]. (* Thanks to Jean-François Alcover. - Wolfdieter Lang, May 31 2013 *) CoefficientList[Series[1/Cyclotomic[4, x], {x, 0, 100}], x] (* Vincenzo Librandi, Apr 03 2014 *) PROG (PARI) {a(n) = real( I^n )} (PARI) {a(n) = kronecker(-4, n+1) } (MAGMA) &cat[ [1, 0, -1, 0]: n in [0..23] ]; // Bruno Berselli, Feb 08 2011 (Maxima) A056594(n) := block(         [1, 0, -1, 0][1+mod(n, 4)] )\$ /* R. J. Mathar, Mar 19 2012 */ CROSSREFS Cf. A049310, A074661, A131852, A002654. Sequence in context: A166698 A059841 * A101455 A091337 A179758 A174888 Adjacent sequences:  A056591 A056592 A056593 * A056595 A056596 A056597 KEYWORD sign,easy AUTHOR Wolfdieter Lang, Aug 04 2000 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.