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 A110921 Numbers n such that n = digit_sum(n)*R(digit_sum(n)) where digit_sum is the sum of digits and R is the digit reversal. 1
 1, 81, 1458, 1729 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS These are special Harshad (or Niven) numbers from A005349. They are mentioned in Simon Singh's book "The Simpsons and their Mathematical Secrets", 2013, Bloomsbury, New York (in the German version "Homers letzter Satz", Hanser, 2013, on p. 238). - Wolfdieter Lang, Feb 05 2014 REFERENCES Tricky Number, Science Illustrated, Jan/Feb 2009, page 80. P. Odifreddi, Il museo dei numeri, Rizzoli, 2014, page 284. LINKS Wikipedia, Masahiko Fujiwara FORMULA k such that A007953(k)*A004086(A007953(k)) = k. - Jonathan Vos Post, Dec 20 2008 EXAMPLE n = 1729: s = 1 + 7 + 2 + 9 = 19, 19*91 = 1729. MATHEMATICA dsrQ[n_]:=Module[{c=Total[IntegerDigits[n]]}, c IntegerReverse[c]==n]; Select[Range, dsrQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 19 2020 *) PROG #!/usr/bin/perl -w use strict; my \$i; my \$j; for \$i ( 1e0 .. 1e6 ) { my \$sum = 0; my \$rev = 0; for \$j ( 1 .. length ( \$i ) ) { \$sum += substr ( \$i, \$j - 1, 1 ); } \$rev = reverse \$sum; print "\$i " if \$sum * \$rev == \$i; } CROSSREFS Cf. A004086, A007953. Sequence in context: A282646 A283542 A237771 * A236867 A203650 A284057 Adjacent sequences:  A110918 A110919 A110920 * A110922 A110923 A110924 KEYWORD nonn,base,fini,full AUTHOR Michael Vang (michael.vang(AT)gmail.com), Sep 22 2005 EXTENSIONS Edited by N. J. A. Sloane, Jan 15 2009, at the suggestion of Klaus Brockhaus Edited by Charles R Greathouse IV, Aug 03 2010 Title corrected by Wolfdieter Lang, Feb 05 2014 STATUS approved

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Last modified January 20 12:33 EST 2021. Contains 340302 sequences. (Running on oeis4.)