cp's OEIS Frontend

A068190 Largest number whose digit product equals n; a(n)=0 if no such number exists, e.g., when n has a prime factor larger than 7; no digit=1 is permitted to avoid an infinite number of solutions.

%I A068190 #14 Dec 08 2018 15:52:06
%S A068190 0,2,3,22,5,32,7,222,33,52,0,322,0,72,53,2222,0,332,0,522,73,0,0,3222,
%T A068190 55,0,333,722,0,532,0,22222,0,0,75,3322,0,0,0,5222,0,732,0,0,533,0,0,
%U A068190 32222,77,552,0,0,0,3332,0,7222,0,0,0,5322,0,0,733,222222,0,0,0,0,0
%N A068190 Largest number whose digit product equals n; a(n)=0 if no such number exists, e.g., when n has a prime factor larger than 7; no digit=1 is permitted to avoid an infinite number of solutions.
%H A068190 David A. Corneth, <a href="/A068190/b068190.txt">Table of n, a(n) for n = 1..10000</a>
%F A068190 If a solution exists, a(n) is the concatenation of prime factors with repetitions and in order of magnitude, otherwise a(n)=0.
%t A068190 Array[If[#[[-1, 1]] > 7, 0, FromDigits@ Reverse@ Flatten@ Map[ConstantArray[#1, #2] & @@ # &, #]] &@ FactorInteger@ # &, 69] /. 1 -> 0 (* _Michael De Vlieger_, Dec 08 2018 *)
%o A068190 (PARI) a(n) = {my(res = []); for(i=2, 9, v = valuation(n, i); if(v > 0, res = concat(vector(v, j, i), res); n/=i^v)); if(n==1,fromdigits(res), 0)} \\ _David A. Corneth_, Jul 31 2017
%Y A068190 Cf. A001222, A002473, A007954, A067734, A068183-A068187, A068189-A068191.
%K A068190 base,nonn
%O A068190 1,2
%A A068190 _Labos Elemer_, Feb 19 2002
%E A068190 a(36) corrected by _David A. Corneth_, Jul 31 2017