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A181739 Smallest semiprime with internal digits = n; or 0 if no such number exists.

%I A181739 #40 Nov 14 2014 09:48:46
%S A181739 106,111,121,133,141,155,161,177,183,194,1101,1111,1121,1133,1141,
%T A181739 1154,1165,1174,1186,1191,1202,1211,1226,1234,1241,1253,1261,1271,
%U A181739 1282,1293,1306,1313,1322,1333,1343,1351,1363,1371,1382,1391,1401,1411,2426
%N A181739 Smallest semiprime with internal digits = n; or 0 if no such number exists.
%C A181739 This is to A069691 as semiprimes A001358 are to primes A000040.
%C A181739 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.
%C A181739 The smallest n such that a(n) = 0 is 20056492. - _Donovan Johnson_, Feb 01 2011
%C A181739 If one or more digits are allowed on both sides of n, the smallest semiprime containing 20056492 is 10200564926 = 2*5100282463.
%H A181739 Alois P. Heinz, <a href="/A181739/b181739.txt">Table of n, a(n) for n = 0..10000</a>
%e A181739 a(23) = 1234 = 2 * 617 has the embedded substring 1"23"4.
%p A181739 a:= proc(n) local i, j, k;
%p A181739       for i to 9 do
%p A181739         for j to 9 do
%p A181739           k:= parse(cat(i, n, j));
%p A181739           if not isprime(k) and add(t[2], t=ifactors(k)[2])=2
%p A181739              then return k fi
%p A181739         od
%p A181739       od; return 0;
%p A181739     end:
%p A181739 seq(a(n), n=0..60);  # _Alois P. Heinz_, Feb 01 2011
%Y A181739 Cf. A001358, A032734, A069691.
%K A181739 nonn,base
%O A181739 0,1
%A A181739 _Jonathan Vos Post_, Jan 31 2011
%E A181739 More terms from _Alois P. Heinz_, Feb 01 2011