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A028835
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Numbers n such that iterated sum of digits of n is a prime.
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5
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| 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 (stefan.steinerberger(AT)gmail.com), Apr 23 2006
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LINKS
| Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
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EXAMPLE
| E.g. 38 -> 3+8 = 11 -> 1+1 = 2 is a prime.
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MATHEMATICA
| Select[Range[200], PrimeQ[Mod[ #, 9]] &] - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Apr 23 2006
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PROG
| (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
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CROSSREFS
| Cf. A010888, A028834, A028843.
Sequence in context: A001742 A073085 A119251 * A028834 A139750 A139749
Adjacent sequences: A028832 A028833 A028834 * A028836 A028837 A028838
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KEYWORD
| nonn,base,easy,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| Extended (and corrected) by Scott Lindhurst (ScottL(AT)alumni.princeton.edu)
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