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A087960
a(n) = (-1)^binomial(n+1,2).
22
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, 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
OFFSET
0,1
COMMENTS
Period 4: repeat [1, -1, -1, 1]. - Joerg Arndt, Feb 14 2016
Also equal to the sign of product(j-i, 1<=j<i<=n+1) = the sign of the Vandermonde determinant for -1, -2, ..., -(n+1).
Hankel transform of A097331, A097332. [Paul Barry, Aug 10 2009]
The Kn22 sums, see A180662, of triangle A108299 equal the terms of this sequence. [Johannes W. Meijer, Aug 14 2011]
REFERENCES
I. S. Gradstein and I. M. Ryshik, Tables of series, products, and integrals, Volume 1, Verlag Harri Deutsch, 1981.
FORMULA
a(n) = (-1)^A000217(n).
a(n) = (-1)^floor((n+1)/2). - Benoit Cloitre and Ray Chandler, Sep 19 2003
G.f.: (1-x)/(1+x^2). - Paul Barry, Aug 10 2009
a(n) = I^(n*(n+1)). - Bruno Berselli, Oct 17 2011
a(n) = Product_{k=1..n} 2*cos(2*k*Pi/(2*n+1)) for n>=0 (for n=0 the empty product is put to 1). See the Gradstein-Ryshik reference, p. 63, 1.396 2. with x = sqrt(-1). - Wolfdieter Lang, Oct 22 2013
a(n) + a(n-2) = 0 for n>1, a(n) = a(n-4) for n>3. - Wesley Ivan Hurt, Jul 07 2016
E.g.f.: cos(x) - sin(x). - Ilya Gutkovskiy, Jul 07 2016
a(n) = Sum_{s=0..n} (-1)^(n-s)*A111125(n, s)*2^s (row polynomials of signed A111125 evaluated at 2). - Wolfdieter Lang, May 02 2021
EXAMPLE
a(1) = -1 since (-1)^binomial(2,2) = (-1)^1 = -1.
G.f. = 1 - x - x^2 + x^3 + x^4 - x^5 - x^6 + x^7 + x^8 - x^9 - x^10 + ...
MAPLE
A087960:=n->(-1)^binomial(n+1, 2): seq(A087960(n), n=0..100); # Wesley Ivan Hurt, Jul 07 2016
MATHEMATICA
(-1)^Binomial[Range[0, 110], 2] (* or *) LinearRecurrence[{0, -1}, {1, 1}, 110] (* Harvey P. Dale, Jul 07 2014 *)
a[ n_] := (-1)^(n (n + 1) / 2); (* Michael Somos, Jul 20 2015 *)
a[ n_] := (-1)^Quotient[ n + 1, 2]; (* Michael Somos, Jul 20 2015 *)
PROG
(PARI) {a(n) = (-1)^((n + 1)\2)}; /* Michael Somos, Jul 20 2015 */
(Haskell)
a087960 n = (-1) ^ (n * (n + 1) `div` 2)
a087960_list = cycle [1, -1, -1, 1] -- Reinhard Zumkeller, Nov 15 2015
(Magma) [(-1)^Binomial(n+1, 2) : n in [0..100]]; // Wesley Ivan Hurt, Jul 07 2016
(Python)
def A087960(n): return -1 if n+1&2 else 1 # Chai Wah Wu, Jan 31 2023
KEYWORD
sign,easy
AUTHOR
W. Edwin Clark, Sep 17 2003
EXTENSIONS
More terms from Benoit Cloitre and Ray Chandler, Sep 19 2003
Offset and Vandermonde formula corrected by R. J. Mathar, Sep 25 2009
STATUS
approved