A190223 - OEIS

%I #24 Sep 14 2020 05:41:56

%S 1,2,3,5,7,11,13,15,17,21,22,23,25,31,33,35,37,51,53,55,71,73,75,77,

%T 111,113,115,121,125,127,131,137,151,155,157,173,175,211,213,217,221,

%U 223,227,231,233,251,253,257,271,275,277,311,313,317,331,337,353

%N Numbers all of whose divisors are numbers whose decimal digits are noncomposite numbers (1,2,3,5,7).

%C Subset of A001742.

%C All terms are obviously odd except for 2 and numbers of the form 2*A004022(k). - _Harvey P. Dale_, May 28 2014 (corrected by _Iain Fox_, Sep 03 2020)

%H Iain Fox, <a href="/A190223/b190223.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from G. C. Greubel)

%e Number 115 is in sequence because all divisors of 115 (1, 5, 23, 115) are numbers whose decimal digits are noncomposite numbers (1,2,3,5,7).

%t ncnQ[n_]:=Module[{digs=Union[Flatten[IntegerDigits/@Divisors[n]]]}, Complement[ digs,{1,2,3,5,7}]=={}]; Select[ Range[ 400],ncnQ] (* _Harvey P. Dale_, May 28 2014 *)

%o (PARI) is(k) = fordiv(k, d, if(setminus(vecsort(digits(d), , 8), [1, 2, 3, 5, 7]) != [], return(0))); 1 \\ _Iain Fox_, Dec 28 2017

%Y Supersequence: A001742.

%Y Cf. A004022, A243534.

%K nonn,base

%O 1,2

%A _Jaroslav Krizek_, May 06 2011

%E More terms from _Harvey P. Dale_, May 28 2014