A042974 - OEIS

A042974

n 1's followed by a 2.

1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1

(list; constant; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

The number 0.121121112... is irrational. - Robert G. Wilson v, Jul 05 2000

Fixed point of the following mapping w. Suppose x(n) takes values in {1,2} and w(n)=[x(1),x(2),...,x(n)]. Next define w(n+1)=[x(1),x(2),...,x(n),x(n-1)] if x(n)=1 and w(n+1)=[x(1),x(2),...,x(n),x(n-1),x(n-1)] if x(n)=2. Then taking x(1)=1 and x(2)=2 we get w(infinity)=A042974 (i.e., x(n)=A042974(n)). - Benoit Cloitre, Jan 11 2013

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..20000

C. V. Eynden, Problem 10439. An irrational mimic of 1/9, Amer. Math. Monthly, 104 (1997), 873.

G. Xiao, Contfrac

Index entries for continued fractions for constants

EXAMPLE

0.1211211121111211111211111121111111211111111211111111121111111111211111...

MATHEMATICA

Table[PadLeft[{2}, n, 1], {n, 2, 20}]//Flatten (* Harvey P. Dale, Sep 11 2019 *)

PROG

(PARI) {a(n) = 1 + issquare(9 + 8*n)}; /* Michael Somos, Jan 12 2000 */

(PARI) { default(realprecision, 20080); x=0; for (n=1, 20000, x=10*x + 1 + issquare(9+8*n)); x/=10^19999; for (n=1, 20000, d=floor(x); x=(x-d)*10; write("b042974.txt", n, " ", d)); } \\ Harry J. Smith, May 08 2009

(Haskell)

a042974 n = a042974_list !! (n-1)

a042974_list = 1 : 2 :

concat (zipWith replicate (tail a042974_list) a042974_list)

-- Reinhard Zumkeller, Dec 08 2011

CROSSREFS

Cf. A042975, A056030 (continued fraction).

Sequence in context: A211983 A173264 A056731 * A235757 A361690 A020906

Adjacent sequences: A042971 A042972 A042973 * A042975 A042976 A042977

KEYWORD

nonn,easy,cons

AUTHOR

N. J. A. Sloane

STATUS

approved