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A066531
Numbers which can be expressed as the product of a number and its reversal in at least two different ways.
15
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
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
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