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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
OFFSET
1,1
LINKS
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
Sequence in context: A174891 A051035 A046349 * A175526 A192506 A091212
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Jan 31 2006
STATUS
approved