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A010690 Period 2: repeat (1,9). 6
1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1, 9, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Digital roots of the nonzero square triangular numbers. - Ant King, Jan 21 2012

Continued fraction expansion of A176019. - R. J. Mathar, Mar 08 2012

Exp( Sum_{n >= 1} a(n-1)*x^n/n ) = 1 + x + 5*x^2 + 5*x^3 + 15*x^4 + 15*x^5 + ... is the o.g.f. for A189976 (taken with an offset of 0). - Peter Bala, Mar 13 2015

LINKS

Table of n, a(n) for n=0..80.

Index entries for linear recurrences with constant coefficients, signature (0,1).

FORMULA

a(n) = -4*(-1)^n+5 =9*(n mod 2)+(n+1) mod 2. - Paolo P. Lava, Oct 20 2006

G.f.: (1+9x)/((1-x)(1+x)). - R. J. Mathar, Nov 21 2011

MATHEMATICA

5+4*(-1)^# &/@Range[81] (* Ant King, Jan 21 2012 *)

PROG

(PARI) a(n)=1; if(n%2==1, 9, 1) \\ Felix Fröhlich, Aug 11 2014

CROSSREFS

Cf. A008592, A189976.

Sequence in context: A200696 A203130 A209050 * A224835 A280375 A067617

Adjacent sequences:  A010687 A010688 A010689 * A010691 A010692 A010693

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified July 22 17:02 EDT 2019. Contains 325225 sequences. (Running on oeis4.)