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A008918 Numbers n such that 4*n = (n written backwards), n > 0. 13
2178, 21978, 219978, 2199978, 21782178, 21999978, 217802178, 219999978, 2178002178, 2197821978, 2199999978, 21780002178, 21978021978, 21999999978, 217800002178, 217821782178, 219780021978, 219978219978, 219999999978, 2178000002178, 2178219782178 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

There are Fibonacci(floor((n-2)/2)) terms with n digits (this is essentially A103609). - Ray Chandler, Oct 12 2017

REFERENCES

D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986.

LINKS

Ray Chandler, Table of n, a(n) for n = 1..10000 (first 200 terms from Vincenzo Librandi)

L. H. Kendrick, Young Graphs: 1089 et al., J. Int. Seq. 18 (2015) 15.9.7.

N. J. A. Sloane, 2178 And All That, Fib. Quart., 52 (2014), 99-120.

FORMULA

Theorem (David W. Wilson): A008918(n) = 2*A001232(n).

MATHEMATICA

Rest@Select[FromDigits /@ Tuples[{0, 198}, 11], IntegerDigits[4*#] == Reverse@IntegerDigits[#] &] (* Arkadiusz Wesolowski, Aug 14 2012 *)

okQ[t_]:=t==Reverse[t]&&First[t]!=0&&Min[Length/@Split[t]]>1; 198#&/@ Flatten[ Table[FromDigits/@Select[Tuples[{0, 1}, n], okQ], {n, 20}]] (* Harvey P. Dale, Jul 03 2013 *)

PROG

(PARI) rev(n) = (eval(concat(Vecrev(Str(n)))));

isok(n) = rev(n) == 4*n; \\ Michel Marcus, Sep 13 2015

CROSSREFS

Cf. A001232, A193434, A008918, A008919, A222814, A222815, A031877.

Sequence in context: A157476 A157853 A072141 * A262792 A035770 A107566

Adjacent sequences:  A008915 A008916 A008917 * A008919 A008920 A008921

KEYWORD

nonn,base

AUTHOR

N. J. A. Sloane.

EXTENSIONS

Corrected and extended by David W. Wilson Aug 15 1996, Dec 15 1997

a(20)-a(21) from Arkadiusz Wesolowski, Aug 14 2012

STATUS

approved

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Last modified November 18 13:22 EST 2018. Contains 317306 sequences. (Running on oeis4.)