OFFSET
1,1
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
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
EXAMPLE
38 -> 3 + 8 = 11 -> 1 + 1 = 2 is a prime.
MATHEMATICA
Select[Range[200], PrimeQ[Mod[ #, 9]] &] (* Stefan Steinerberger, Apr 23 2006 *)
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
CROSSREFS
KEYWORD
nonn,base,easy,nice
AUTHOR
EXTENSIONS
Extended (and corrected) by Scott Lindhurst (ScottL(AT)alumni.princeton.edu)
STATUS
approved