cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A082994 Numbers n such that all the following properties hold: (i) n*reverse(n) is a square; (ii) n != reverse(n); (iii) n and reverse(n) are not both squares; and (iv) n and reverse(n) have the same number of digits.

Original entry on oeis.org

288, 528, 768, 825, 867, 882, 1584, 2178, 4851, 8712, 10989, 13104, 14544, 15984, 20808, 21978, 26208, 27648, 27848, 36828, 40131, 44541, 48139, 48951, 49686, 57399, 68694, 80262, 80802, 82863, 84672, 84872, 87912, 93184, 98901, 99375
Offset: 1

Views

Author

Jason Earls, May 29 2003

Keywords

Comments

These numbers are counterexamples to the following conjecture given in the Ogilvy-Anderson reference: "When an integer and its reversal are unequal, their product is never a square except when both are squares." This sequence excludes terms like 2200, i.e. 2200*22 = 48400.
Contains x*(10^k+1) for k >= 3 with x in {144, 169, 288, 441, 528, 768, 825, 867, 882, 961}. - Robert Israel, Jun 11 2018
A035090 U {this sequence} = A062917, with empty intersection. - Bernard Schott, Jan 04 2019

Examples

			a(5) = 867 because 867 * 768 = 665856 = 816^2.
		

References

  • C. Stanley Ogilvy and John T. Anderson, Excursions in Number Theory, Oxford University Press, NY. (1966), pp. 88-89.
  • J. Earls, Mathematical Bliss, Pleroma Publications, 2009, pages 82-83. ASIN: B002ACVZ6O [From Jason Earls, Nov 22 2009]

Crossrefs

Programs

  • Maple
    revdigs:= proc(n) local L;
      L:= convert(n,base,10);
      add(L[-i]*10^(i-1),i=1..nops(L))
    end proc:
    filter:= proc(n) local r;
      if issqr(n) then return false fi;
      r:= revdigs(n);
      r <> n and issqr(r*n) and not issqr(r);
    end proc:
    select(filter, [seq(seq(10*i+j,j=1..9),i=1..10^4)]); # Robert Israel, Jun 11 2018
  • Mathematica
    Select[Range[10^5], And[UnsameQ @@ {#1, #2}, IntegerQ@ Sqrt[#1 #2], AllTrue[{#1, #2}, ! IntegerQ@ Sqrt@ # &], SameQ @@ (IntegerLength@ {#1, #2})] & @@ {#, IntegerReverse@ #} &] (* Michael De Vlieger, Jan 04 2019 *)

Extensions

Name clarified by Bernard Schott, Jan 04 2019