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

1,1

Links

Table of n, a(n) for n=1..60.

Example

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

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 *)

Crossrefs

Cf. A000040, A084317, A084318.

Sequence in context: A079142 A062002 A090419 * A028958 A200878 A344030

Adjacent sequences: A209796 A209797 A209798 * A209800 A209801 A209802

Keyword

nonn,base,less

Author

Michel Lagneau, Mar 13 2012

Status

approved