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A169998 a(0)=1, a(1)=1; thereafter a(n) = -a(n-1) - 2*a(n-2). 5
1, 1, -3, 1, 5, -7, -3, 17, -11, -23, 45, 1, -91, 89, 93, -271, 85, 457, -627, -287, 1541, -967, -2115, 4049, 181, -8279, 7917, 8641, -24475, 7193, 41757, -56143, -27371, 139657, -84915, -194399, 364229, 24569, -753027, 703889, 802165, -2209943, 605613, 3814273, -5025499, -2603047 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Cassels, following Nagell, shows that a(n) = +- 1 only for n = 1, 2, 3, 5, 13.

The sequences A001607, A077020, A107920, A167433, A169998 are all essentially the same except for signs.

REFERENCES

J. W. S. Cassels, Local Fields, Cambridge, 1986, see p. 67.

LINKS

Table of n, a(n) for n=0..45.

F. Beukers, The multiplicity of binary recurrences, Compositio Mathematica, Tome 40 (1980) no. 2 , p. 251-267. See Theorem 2 p. 259.

M. Mignotte, Propriétés arithmétiques des suites récurrentes, Besançon, 1988-1989, see p. 14. In French.

Index entries for linear recurrences with constant coefficients, signature (-1,-2).

FORMULA

G.f.: ( 1+2*x ) / ( 1+x+2*x^2 ). - R. J. Mathar, Jul 14 2011

MAPLE

f:=proc(n) option remember; if n <= 1 then 1 else -f(n-1)-2*f(n-2); fi; end;

PROG

(PARI) a(n)=([0, 1; -2, -1]^n*[1; 1])[1, 1] \\ Charles R Greathouse IV, Jun 11 2015

CROSSREFS

Sequence in context: A167433 A077020 A107920 * A326729 A171998 A159285

Adjacent sequences:  A169995 A169996 A169997 * A169999 A170000 A170001

KEYWORD

sign,easy

AUTHOR

N. J. A. Sloane, Aug 29 2010

STATUS

approved

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Last modified July 2 14:41 EDT 2020. Contains 335401 sequences. (Running on oeis4.)