OFFSET
1,1
COMMENTS
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
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..1452
EXAMPLE
217 is in the sequence because 2 * 1 * 7 = 14 = 2 * 7 which is a semiprime.
312 is in the sequence because 3 * 1 * 2 = 6 = 2 * 3 which is a semiprime.
MAPLE
dmax:= 4: # to get all terms with up to d digits
A:= NULL:
for d from 1 to dmax do
for j from 1 to d do
for xj in [4, 6, 9] do
A:= A, (10^d-1)/9 + (xj-1)*10^(j-1);
od od:
for ij in combinat[choose](d, 2) do
for xi in [2, 3, 5, 7] do
for xj in [2, 3, 5, 7] do
A:= A, (10^d-1)/9 + (xi-1)*10^(ij[1]-1) + (xj-1)*10^(ij[2]-1);
od od od:
od:
{A}; # Robert Israel, Jul 20 2014
MATHEMATICA
Select[Range[500], PrimeOmega[(Times @@ IntegerDigits[#])] == 2 &]
PROG
(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", ")))
for(d=1, 4, f(0, 0, d)) \\ f(0, 0, d) prints d-digit terms. Jens Kruse Andersen, Jul 21 2014
CROSSREFS
KEYWORD
nonn,base
AUTHOR
K. D. Bajpai, Jul 20 2014
STATUS
approved