cp's OEIS Frontend

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

%I A209799 #16 Apr 10 2023 16:58:06
%S A209799 4,6,8,9,12,16,18,21,22,24,25,27,32,33,36,39,44,46,48,49,51,54,58,63,
%T A209799 64,66,70,72,81,82,88,92,93,96,99,108,111,115,116,117,121,125,128,132,
%U A209799 133,140,141,142,144,147,153,154,159,162,164,165,166,169,176,177
%N A209799 Composite numbers n such that the concatenation of the digits of the prime divisors of n is a prime number.
%e A209799 70 is in the sequence because the prime divisors of 70 are {2,5,7} and 257 is prime.
%p A209799 read("transforms") ;
%p A209799 isA209799 := proc(n)
%p A209799     local pdivs ;
%p A209799     if isprime(n) or n < 4 then
%p A209799         return false;
%p A209799     end if;
%p A209799     pdivs := sort(convert(numtheory[factorset](n),list)) ;
%p A209799     isprime(digcatL(pdivs)) ;
%p A209799 end proc:
%p A209799 for n from 4 to 200 do
%p A209799         if isA209799(n) then printf("%d,",n) ;
%p A209799         end if;
%p A209799 end do: # _R. J. Mathar_, Mar 19 2012
%t A209799 Select[Range[200],CompositeQ[#]&&PrimeQ[FromDigits[Flatten[ IntegerDigits/@ FactorInteger[#] [[;;,1]]]]]&] (* _Harvey P. Dale_, Apr 10 2023 *)
%Y A209799 Cf. A000040, A084317, A084318.
%K A209799 nonn,base,less
%O A209799 1,1
%A A209799 _Michel Lagneau_, Mar 13 2012