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A115683 Numbers that are the product of 2 palindromes greater than 1. 17
4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 27, 28, 30, 32, 33, 35, 36, 40, 42, 44, 45, 48, 49, 54, 55, 56, 63, 64, 66, 72, 77, 81, 88, 99, 110, 121, 132, 154, 165, 176, 198, 202, 220, 222, 231, 242, 262, 264, 275, 282, 297, 302, 303, 308, 322, 330, 333 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

262 = 2 * 131.

264 = 6 * 11.

275 = 5 * 55.

MAPLE

revdigs:= proc(n) local L, i;

  L:= convert(n, base, 10);

  add(L[-i]*10^(i-1), i=1..nops(L))

end proc:

N:=4; # to get terms <= 2*10^N

Pals:= $2..9:

for d from 2 to N do

  if d::even then

    m:= d/2;

    Pals:= Pals, seq(n*10^m + revdigs(n), n=10^(m-1)..10^m-1);

  else

    m:= (d-1)/2;

    Pals:= Pals, seq(seq(n*10^(m+1)+y*10^m+revdigs(n), y=0..9), n=10^(m-1)..10^m-1);

  fi

od:

Pals:= {Pals}: nP:= nops(Pals):

P2:= select(`<`, {seq(seq(Pals[i]*Pals[j], j=1..i), i=1..nP)}, 2*10^N):

sort(convert(P2, list)); # Robert Israel, Mar 16 2020

MATHEMATICA

pal = Select[ Range[2, 400], # == FromDigits@ Reverse@ IntegerDigits@ # &]; Select[Union[Times @@@ Tuples[pal, 2]], # <= 400 &] (* Giovanni Resta, Jun 20 2016 *)

CROSSREFS

Cf. A002113, A141322.

Sequence in context: A174891 A051035 A046349 * A175526 A192506 A091212

Adjacent sequences:  A115680 A115681 A115682 * A115684 A115685 A115686

KEYWORD

nonn,base

AUTHOR

Giovanni Resta, Jan 31 2006

STATUS

approved

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Last modified June 13 00:57 EDT 2021. Contains 344980 sequences. (Running on oeis4.)