A028835 Numbers whose iterated sum of digits is a prime.

2, 3, 5, 7, 11, 12, 14, 16, 20, 21, 23, 25, 29, 30, 32, 34, 38, 39, 41, 43, 47, 48, 50, 52, 56, 57, 59, 61, 65, 66, 68, 70, 74, 75, 77, 79, 83, 84, 86, 88, 92, 93, 95, 97, 101, 102, 104, 106, 110, 111, 113, 115, 119, 120, 122, 124, 128, 129, 131, 133, 137, 138, 140, 142, 146, 147, 149, 151, 155, 156

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• base
• easy
• nice

Comments

Also numbers k such that k mod 9 is an element of {2,3,5,7}. Hence as n tends to infinity, a(n)/n converges to 9/4 quite rapidly. - Stefan Steinerberger, Apr 23 2006

Examples

			38 -> 3 + 8 = 11 -> 1 + 1 = 2 is a prime.
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A010888, A028834, A028843.

Programs

Haskell

import Data.List (findIndices)
a028835 n = a028835_list !! (n-1)
a028835_list = findIndices (`elem` [2,3,5,7]) $ map a010888 [0..]
-- Reinhard Zumkeller, Oct 21 2011

Mathematica

Select[Range[200], PrimeQ[Mod[ #, 9]] &] (* Stefan Steinerberger, Apr 23 2006 *)

Extensions

Extended (and corrected) by Scott Lindhurst (ScottL(AT)alumni.princeton.edu)