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A083815
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Semiprimes whose prime factors are distinct and the reversal of one factor is equal to the other.
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7
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403, 1207, 2701, 7663, 35143, 75007, 117907, 127087, 140209, 173809, 197209, 247021, 257821, 342127, 382387, 643063, 692443, 743623, 1226221, 1341331, 1626151, 1698661, 1739161, 2073991, 2138791, 2528611, 2561011, 3321133
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OFFSET
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1,1
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COMMENTS
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LINKS
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EXAMPLE
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a(2)= 1207 = 17 * 71.
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MAPLE
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revdigs:= proc(n) local i, L;
L:= convert(n, base, 10);
add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
f:= proc(p) local r;
if not isprime(p) then return NULL fi;
r:= revdigs(p);
if r > p and isprime(r) then r*p fi
end proc:
sort(map(f, [seq(i, i=13..9999, 2)])); # Robert Israel, Dec 26 2018
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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