A181739 - OEIS

%I #40 Nov 14 2014 09:48:46

%S 106,111,121,133,141,155,161,177,183,194,1101,1111,1121,1133,1141,

%T 1154,1165,1174,1186,1191,1202,1211,1226,1234,1241,1253,1261,1271,

%U 1282,1293,1306,1313,1322,1333,1343,1351,1363,1371,1382,1391,1401,1411,2426

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

%C This is to A069691 as semiprimes A001358 are to primes A000040.

%C 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 The smallest n such that a(n) = 0 is 20056492. - _Donovan Johnson_, Feb 01 2011

%C If one or more digits are allowed on both sides of n, the smallest semiprime containing 20056492 is 10200564926 = 2*5100282463.

%H Alois P. Heinz, <a href="/A181739/b181739.txt">Table of n, a(n) for n = 0..10000</a>

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

%p a:= proc(n) local i, j, k;

%p       for i to 9 do

%p         for j to 9 do

%p           k:= parse(cat(i, n, j));

%p           if not isprime(k) and add(t[2], t=ifactors(k)[2])=2

%p              then return k fi

%p         od

%p       od; return 0;

%p     end:

%p seq(a(n), n=0..60);  # _Alois P. Heinz_, Feb 01 2011

%Y Cf. A001358, A032734, A069691.

%K nonn,base

%O 0,1

%A _Jonathan Vos Post_, Jan 31 2011

%E More terms from _Alois P. Heinz_, Feb 01 2011