

A181739


Smallest semiprime with internal digits = n; or 0 if no such number exists.


1



106, 111, 121, 133, 141, 155, 161, 177, 183, 194, 1101, 1111, 1121, 1133, 1141, 1154, 1165, 1174, 1186, 1191, 1202, 1211, 1226, 1234, 1241, 1253, 1261, 1271, 1282, 1293, 1306, 1313, 1322, 1333, 1343, 1351, 1363, 1371, 1382, 1391, 1401, 1411, 2426
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OFFSET

0,1


COMMENTS

By placing one digit on both sides of n (1..9 on the left and on the right) one gets 81 different numbers that might be semiprimes. If none of these numbers is a semiprime then a(n) = 0.
The smallest n such that a(n) = 0 is 20056492.  Donovan Johnson, Feb 01 2011
If one or more digits are allowed on both sides of n, the smallest semiprime containing 20056492 is 10200564926 = 2*5100282463.


LINKS



EXAMPLE

a(23) = 1234 = 2 * 617 has the embedded substring 1"23"4.


MAPLE

a:= proc(n) local i, j, k;
for i to 9 do
for j to 9 do
k:= parse(cat(i, n, j));
if not isprime(k) and add(t[2], t=ifactors(k)[2])=2
then return k fi
od
od; return 0;
end:


CROSSREFS



KEYWORD

nonn,base


AUTHOR



EXTENSIONS



STATUS

approved



