A199980 - OEIS

%I #18 Nov 05 2020 19:22:11

%S 2,37,43,73,137,173,211,223,317,367,389,431,673,827,839,929,983,1223,

%T 1279,1297,1367,1447,1621,1637,1693,1999,2111,2161,2179,2213,2269,

%U 2339,2393,2663,2699,2719,2791,2917,2969,2971,3167,3169,3221,3329,3463,3499,3617

%N Primes whose multiplicative digital root is 2.

%C Complement of A199981 with respect to A034049, numbers whose multiplicative digital root is 2.

%H Robert Israel, <a href="/A199980/b199980.txt">Table of n, a(n) for n = 1..10000</a>

%e Prime 389 is in sequence because 3*8*9=216, 2*1*6 =12, 1*2=2.

%p mdr:= proc(n) option remember;

%p   local t;

%p   t:= convert(convert(n,base,10),`*`);

%p   if t < 10  then t else procname(t) fi

%p end proc:

%p select(t -> mdr(t) = 2 and isprime(t), [2, seq(i,i=3..10000,2)]); # _Robert Israel_, Nov 05 2020

%t t = {}; n = 0; While[Length[t] < 100, n = NextPrime[n]; s = n; While[s >= 10, s = Times @@ IntegerDigits[s]]; If[s == 2, AppendTo[t, n]]]; t (* _T. D. Noe_, Nov 15 2011 *)

%t Select[Prime[Range[600]],FixedPoint[Times@@IntegerDigits[#]&,#]==2&] (* _Harvey P. Dale_, Mar 28 2012 *)

%Y Cf. A199981 (nonprime numbers whose multiplicative digital root is 2).

%Y Includes A107612.

%Y Cf. A031347, A034049.

%K nonn,base

%O 1,1

%A _Jaroslav Krizek_, Nov 13 2011