cp's OEIS Frontend

A052021 Sum of digits of n is the largest prime factor of n.

%I A052021 #20 Nov 30 2015 09:39:10
%S A052021 2,3,5,7,12,50,70,308,320,364,476,500,605,700,704,715,832,935,1088,
%T A052021 1183,1547,1729,2401,2584,2618,2704,2926,3080,3200,3536,3640,3952,
%U A052021 4225,4760,4784,4913,5000,5491,5525,5819,5831,6050,6175,6517,6647,7000,7040,7150
%N A052021 Sum of digits of n is the largest prime factor of n.
%H A052021 Reinhard Zumkeller, <a href="/A052021/b052021.txt">Table of n, a(n) for n = 1..1000</a>
%F A052021 {n: A007953(n) = A006530(n)}. - _R. J. Mathar_, May 30 2010
%e A052021 13685 has sum of digits '23' and 13685 = 5*7*17*'23'.
%p A052021 A007953 := proc(n) add(d,d=convert(n,base,10)) ; end proc:
%p A052021 A006530 := proc(n) numtheory[factorset](n) ; max(op(%)) ; end proc:
%p A052021 for n from 1 to 8000 do if A007953(n) = A006530(n) then printf("%d,",n) ; end if; end do: # _R. J. Mathar_, May 30 2010
%t A052021 Select[Range[2,8000],FactorInteger[#][[-1,1]]==Total[IntegerDigits[#]]&] (* _Harvey P. Dale_, Oct 17 2012 *)
%o A052021 (Haskell)
%o A052021 a052021 n = a052021_list !! (n-1)
%o A052021 a052021_list = tail $ filter (\x -> a007953 x == a006530 x) [1..]
%o A052021 -- _Reinhard Zumkeller_, Nov 06 2011
%Y A052021 Cf. A052018, A052019, A052020, A052022, A007953, A005349, A028834.
%K A052021 nonn,base,nice
%O A052021 1,1
%A A052021 _Patrick De Geest_, Nov 15 1999
%E A052021 Single-digit primes added by _R. J. Mathar_, May 30 2010
%E A052021 Offset corrected by _Reinhard Zumkeller_, Nov 05 2011