A115683 Numbers that are the product of 2 palindromes greater than 1.

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

Offset: 1

Author

Giovanni Resta, Jan 31 2006

Keywords

• nonn
• base

Examples

			262 = 2 * 131.
264 = 6 * 11.
275 = 5 * 55.
		

Links

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

Crossrefs

Cf. A002113, A141322.

Programs

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 *)