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

4, 6, 9, 14, 16, 19, 22, 23, 25, 27, 32, 33, 35, 37, 41, 52, 53, 55, 57, 61, 72, 73, 75, 77, 91, 114, 116, 119, 122, 123, 125, 127, 132, 133, 135, 137, 141, 152, 153, 155, 157, 161, 172, 173, 175, 177, 191, 212, 213, 215, 217, 221, 231, 251, 271, 312, 313, 315

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

Cf. A001358, A046703, A007954.

Sequence in context: A108634 A135355 A350069 * A050940 A190434 A310671

Adjacent sequences: A245380 A245381 A245382 * A245384 A245385 A245386

Keyword

nonn,base

Author

K. D. Bajpai, Jul 20 2014

Status

approved