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

2, 3, 5, 7, 12, 50, 70, 308, 320, 364, 476, 500, 605, 700, 704, 715, 832, 935, 1088, 1183, 1547, 1729, 2401, 2584, 2618, 2704, 2926, 3080, 3200, 3536, 3640, 3952, 4225, 4760, 4784, 4913, 5000, 5491, 5525, 5819, 5831, 6050, 6175, 6517, 6647, 7000, 7040, 7150

Offset: 1

Author

Patrick De Geest, Nov 15 1999

Keywords

• nonn
• base
• nice

Examples

			13685 has sum of digits '23' and 13685 = 5*7*17*'23'.
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A052018, A052019, A052020, A052022, A007953, A005349, A028834.

Programs

Haskell

a052021 n = a052021_list !! (n-1)
a052021_list = tail $ filter (\x -> a007953 x == a006530 x) [1..]
-- Reinhard Zumkeller, Nov 06 2011

Maple

A007953 := proc(n) add(d,d=convert(n,base,10)) ; end proc:
A006530 := proc(n) numtheory[factorset](n) ; max(op(%)) ; end proc:
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

Mathematica

Select[Range[2,8000],FactorInteger[#][[-1,1]]==Total[IntegerDigits[#]]&] (* Harvey P. Dale, Oct 17 2012 *)

Formula

{n: A007953(n) = A006530(n)}. - R. J. Mathar, May 30 2010

Extensions

Single-digit primes added by R. J. Mathar, May 30 2010

Offset corrected by Reinhard Zumkeller, Nov 05 2011