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%I #19 Aug 28 2021 19:08:35
%S 2520,4030,5740,7360,7650,9760,10080,12070,13000,14580,14620,16120,
%T 17290,18550,19440,22680,22960,24300,25200,26680,27010,29440,31540,
%U 34780,36270,36400,40300,40320,42750,46060,49300,50920,56050,57400
%N Numbers which can be expressed as the product of a number and its reversal in at least two different ways.
%C The digital root of these terms is always 1,4,7 or 9.
%D S. S. Gupta, EPRNs, Science Today, Feb. 1987, India.
%H Hans Havermann, <a href="/A066531/b066531.txt">Table of n, a(n) for n = 1..10000</a>
%H Shyam Sunder Gupta, <a href="http://www.shyamsundergupta.com/eporns.htm">EPRN Numbers</a>
%e 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.
%t 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]] &]]]]
%t 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 *)
%K base,nonn
%O 1,1
%A _Robert G. Wilson v_, Jan 06 2002
%E Edited by _Hans Havermann_, Feb 11 2012
%E Definition rewritten by _N. J. A. Sloane_, Aug 01 2019
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