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A028835 Numbers n such that iterated sum of digits of n is a prime. 6

%I

%S 2,3,5,7,11,12,14,16,20,21,23,25,29,30,32,34,38,39,41,43,47,48,50,52,

%T 56,57,59,61,65,66,68,70,74,75,77,79,83,84,86,88,92,93,95,97,101,102,

%U 104,106,110,111,113,115,119,120,122,124,128,129,131,133,137,138,140,142,146,147,149,151,155,156

%N Numbers n such that iterated sum of digits of n is a prime.

%C Also numbers n such that n modulo 9 is an element of {2,3,5,7}. Hence as n tends to infinity, a(n)/n converges against 9/4 quite rapidly. - _Stefan Steinerberger_, Apr 23 2006

%H Reinhard Zumkeller, <a href="/A028835/b028835.txt">Table of n, a(n) for n = 1..10000</a>

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

%t Select[Range[200], PrimeQ[Mod[ #, 9]] &] (* _Stefan Steinerberger_, Apr 23 2006 *)

%o (Haskell)

%o import Data.List (findIndices)

%o a028835 n = a028835_list !! (n-1)

%o a028835_list = findIndices (`elem` [2,3,5,7]) $ map a010888 [0..]

%o -- _Reinhard Zumkeller_, Oct 21 2011

%Y Cf. A010888, A028834, A028843.

%K nonn,base,easy,nice

%O 1,1

%A _N. J. A. Sloane_

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

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Last modified October 23 11:19 EDT 2019. Contains 328345 sequences. (Running on oeis4.)