login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 26 15:50 EDT 2022. Contains 357000 sequences. (Running on oeis4.)