OFFSET
1,2
COMMENTS
Equivalently, numbers whose decimal representation consists of digits from the set {1,3,9}.
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
EXAMPLE
a(9)=39 is in the sequence because 3*9=3^3.
MATHEMATICA
Select[Range[2000], IntegerQ[Log[3, Times @@ (IntegerDigits[#])]] &]
PROG
(Haskell)
a174813 n = a174813_list !! (n-1)
a174813_list = f [1] where
f ds = foldr (\d v -> 10 * v + d) 0 ds : f (s ds)
s [] = [1]; s (9:ds) = 1 : s ds; s (d:ds) = 3*d : ds
-- Reinhard Zumkeller, Jan 13 2014
(Python)
from sympy import integer_log
def A174813(n):
m = integer_log(k:=(n<<1)+1, 3)[0]
return sum(3**((k-3**m)//(3**j<<1)%3)*10**j for j in range(m)) # Chai Wah Wu, Jun 28 2025
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Michel Lagneau, Dec 01 2010
STATUS
approved
