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

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

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 * A258540 A159214 A224542

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