cp's OEIS Frontend

A245383 Numbers n whose product of decimal digits is a semiprime.

%I A245383 #15 Jul 21 2014 17:38:30
%S A245383 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 A245383 91,114,116,119,122,123,125,127,132,133,135,137,141,152,153,155,157,
%U A245383 161,172,173,175,177,191,212,213,215,217,221,231,251,271,312,313,315
%N A245383 Numbers n whose product of decimal digits is a semiprime.
%C A245383 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 A245383 K. D. Bajpai, <a href="/A245383/b245383.txt">Table of n, a(n) for n = 1..1452</a>
%e A245383 217 is in the sequence because 2 * 1 * 7 = 14 = 2 * 7 which is a semiprime.
%e A245383 312 is in the sequence because 3 * 1 * 2 = 6 = 2 * 3 which is a semiprime.
%p A245383 dmax:= 4: # to get all terms with up to d digits
%p A245383 A:= NULL:
%p A245383 for d from 1 to dmax do
%p A245383   for j from 1 to d do
%p A245383      for xj in [4,6,9] do
%p A245383         A:= A,(10^d-1)/9 + (xj-1)*10^(j-1);
%p A245383   od od:
%p A245383   for ij in combinat[choose](d,2) do
%p A245383     for xi in [2,3,5,7] do
%p A245383       for xj in [2,3,5,7] do
%p A245383         A:= A,(10^d-1)/9 + (xi-1)*10^(ij[1]-1) + (xj-1)*10^(ij[2]-1);
%p A245383   od od od:
%p A245383 od:
%p A245383 {A}; # _Robert Israel_, Jul 20 2014
%t A245383 Select[Range[500], PrimeOmega[(Times @@ IntegerDigits[#])] == 2 &]
%o A245383 (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 A245383 for(d=1, 4, f(0,0,d)) \\ f(0,0,d) prints d-digit terms. _Jens Kruse Andersen_, Jul 21 2014
%Y A245383 Cf. A001358, A046703, A007954.
%K A245383 nonn,base
%O A245383 1,1
%A A245383 _K. D. Bajpai_, Jul 20 2014