OFFSET
1,1
COMMENTS
a(n) is congruent to 0, 4, 6 or 8 mod 9. - Robert Israel, Aug 12 2014
LINKS
Derek Orr, Table of n, a(n) for n = 1..10000
EXAMPLE
78 is a term because 78, 7+8 = 15, and 1+5 = 6 are composite.
MAPLE
filter:= proc(n) local x;
x:= n;
do
if isprime(x) then return false fi;
if x < 10 then return (x > 1) fi;
x:= convert(convert(x, base, 10), `+`);
od:
end proc;
select(filter, [$4..1000]); # Robert Israel, Aug 12 2014
MATHEMATICA
okQ[n_] := n > 1 && !PrimeQ[n] && (n < 10 || okQ@ Total@ IntegerDigits@ n); Select[Range@168, okQ] (* Giovanni Resta, Aug 05 2013 *)
cnQ[n_]:=AllTrue[NestWhileList[Total[IntegerDigits[#]]&, n, #>9&], CompositeQ]; Select[Range[210], cnQ] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Mar 25 2016 *)
PROG
(PARI)
forcomposite(n=1, 500, s=sumdigits(n); while(s>9&&!isprime(s)&&s!=1, s=sumdigits(s)); if(!isprime(s)&&s!=1, print1(n, ", "))) \\ Derek Orr, Aug 12 2014
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Derek Orr, Aug 02 2013
STATUS
approved