A066531 - OEIS

A066531

Numbers which can be expressed as the product of a number and its reversal in at least two different ways.

2520, 4030, 5740, 7360, 7650, 9760, 10080, 12070, 13000, 14580, 14620, 16120, 17290, 18550, 19440, 22680, 22960, 24300, 25200, 26680, 27010, 29440, 31540, 34780, 36270, 36400, 40300, 40320, 42750, 46060, 49300, 50920, 56050, 57400

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OFFSET

1,1

COMMENTS

The digital root of these terms is always 1,4,7 or 9.

REFERENCES

S. S. Gupta, EPRNs, Science Today, Feb. 1987, India.

LINKS

Hans Havermann, Table of n, a(n) for n = 1..10000

Shyam Sunder Gupta, EPRN Numbers

Shyam Sunder Gupta, Equal Product of Reversible Numbers (EPRN), Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 12, 353-365.

EXAMPLE

4030 = 130 * 031 = 310 * 013, 144648 = 861 * 168 = 492 * 294, 185472 = 672 * 276 = 384 * 483, 9949716 = 2583 * 3852 = 1476 * 6741, 16746912 = 2556 * 6552 = 4473 * 3744, etc.

MATHEMATICA

f[n_] := (r = FromDigits[Reverse[IntegerDigits[n]]]; If[n >= r, n*r, 0]); s = Sort[DeleteCases[Table[f[i], {i, 10^4}], 0]]; Union[s[[Select[Range[Length[s]] - 1, s[[#]] == s[[# + 1]] &]]]]

Take[Select[Tally[Table[If[n<IntegerReverse[n], Nothing, n IntegerReverse[ n]], {n, 5000}]], #[[2]]>1&][[All, 1]]//Union, 40] (* Harvey P. Dale, Aug 28 2021 *)

CROSSREFS

Sequence in context: A068547 A094515 A179700 * A385511 A258540 A159214

Adjacent sequences: A066528 A066529 A066530 * A066532 A066533 A066534

KEYWORD

base,nonn

AUTHOR

Robert G. Wilson v, Jan 06 2002

EXTENSIONS

Edited by Hans Havermann, Feb 11 2012

Definition rewritten by N. J. A. Sloane, Aug 01 2019

STATUS

approved