

A245383


Numbers n whose product of decimal digits is a semiprime.


1



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
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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



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^d1)/9 + (xj1)*10^(j1);
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^d1)/9 + (xi1)*10^(ij[1]1) + (xj1)*10^(ij[2]1);
od od od:
od:


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), d1))), if(b==2, print1(n", ")))


CROSSREFS



KEYWORD

nonn,base


AUTHOR



STATUS

approved



