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A115683
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Numbers that are the product of 2 palindromes greater than 1.
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17
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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
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OFFSET
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1,1
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LINKS
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EXAMPLE
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262 = 2 * 131.
264 = 6 * 11.
275 = 5 * 55.
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MAPLE
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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):
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MATHEMATICA
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pal = Select[ Range[2, 400], # == FromDigits@ Reverse@ IntegerDigits@ # &]; Select[Union[Times @@@ Tuples[pal, 2]], # <= 400 &] (* Giovanni Resta, Jun 20 2016 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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