A209799 Composite numbers n such that the concatenation of the digits of the prime divisors of n is a prime number.

4, 6, 8, 9, 12, 16, 18, 21, 22, 24, 25, 27, 32, 33, 36, 39, 44, 46, 48, 49, 51, 54, 58, 63, 64, 66, 70, 72, 81, 82, 88, 92, 93, 96, 99, 108, 111, 115, 116, 117, 121, 125, 128, 132, 133, 140, 141, 142, 144, 147, 153, 154, 159, 162, 164, 165, 166, 169, 176, 177

Offset: 1

Author

Michel Lagneau, Mar 13 2012

Keywords

• nonn
• base
• less

Examples

			70 is in the sequence because the prime divisors of 70 are {2,5,7} and 257 is prime.
		

Crossrefs

Cf. A000040, A084317, A084318.

Programs

Maple

read("transforms") ;
isA209799 := proc(n)
    local pdivs ;
    if isprime(n) or n < 4 then
        return false;
    end if;
    pdivs := sort(convert(numtheory[factorset](n),list)) ;
    isprime(digcatL(pdivs)) ;
end proc:
for n from 4 to 200 do
        if isA209799(n) then printf("%d,",n) ;
        end if;
end do: # R. J. Mathar, Mar 19 2012

Mathematica

Select[Range[200],CompositeQ[#]&&PrimeQ[FromDigits[Flatten[ IntegerDigits/@ FactorInteger[#] [[;;,1]]]]]&] (* Harvey P. Dale, Apr 10 2023 *)