A245383 - OEIS

%I #15 Jul 21 2014 17:38:30

%S 4,6,9,14,16,19,22,23,25,27,32,33,35,37,41,52,53,55,57,61,72,73,75,77,

%T 91,114,116,119,122,123,125,127,132,133,135,137,141,152,153,155,157,

%U 161,172,173,175,177,191,212,213,215,217,221,231,251,271,312,313,315

%N Numbers n whose product of decimal digits is a semiprime.

%C n either has one digit 4, 6 or 9 or two digits in {2,3,5,7}, all other digits being 1. - _Robert Israel_, Jul 20 2014

%H K. D. Bajpai, <a href="/A245383/b245383.txt">Table of n, a(n) for n = 1..1452</a>

%e 217 is in the sequence because 2 * 1 * 7 = 14 = 2 * 7 which is a semiprime.

%e 312 is in the sequence because 3 * 1 * 2 = 6 = 2 * 3 which is a semiprime.

%p dmax:= 4: # to get all terms with up to d digits

%p A:= NULL:

%p for d from 1 to dmax do

%p   for j from 1 to d do

%p      for xj in [4,6,9] do

%p         A:= A,(10^d-1)/9 + (xj-1)*10^(j-1);

%p   od od:

%p   for ij in combinat[choose](d,2) do

%p     for xi in [2,3,5,7] do

%p       for xj in [2,3,5,7] do

%p         A:= A,(10^d-1)/9 + (xi-1)*10^(ij[1]-1) + (xj-1)*10^(ij[2]-1);

%p   od od od:

%p od:

%p {A}; # _Robert Israel_, Jul 20 2014

%t Select[Range[500], PrimeOmega[(Times @@ IntegerDigits[#])] == 2 &]

%o (PARI) f(n,b,d) = if(d, for(i=1, 9, if(b+bigomega(i)<=2, f(10*n+i, b+bigomega(i), d-1))), if(b==2, print1(n", ")))

%o for(d=1, 4, f(0,0,d)) \\ f(0,0,d) prints d-digit terms. _Jens Kruse Andersen_, Jul 21 2014

%Y Cf. A001358, A046703, A007954.

%K nonn,base

%O 1,1

%A _K. D. Bajpai_, Jul 20 2014