A143967 Numbers containing only digits 3 or 7 in decimal representation.

3, 7, 33, 37, 73, 77, 333, 337, 373, 377, 733, 737, 773, 777, 3333, 3337, 3373, 3377, 3733, 3737, 3773, 3777, 7333, 7337, 7373, 7377, 7733, 7737, 7773, 7777, 33333, 33337, 33373, 33377, 33733, 33737, 33773, 33777, 37333, 37337, 37373, 37377

Offset: 1

Author

Reinhard Zumkeller, Sep 06 2008

Keywords

• base
• nonn

Comments

See A020463 for primes.

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A004086, A020463, A032810.

Programs

Haskell

a143967 = f 0 . (+ 1) where
   f y 1 = a004086 y
   f y x = f (10 * y + 3 + 4 * r) x' where (x', r) = divMod x 2
-- Reinhard Zumkeller, Mar 18 2015

Mathematica

Table[FromDigits/@Tuples[{3,7},n],{n,5}]//Flatten (* Harvey P. Dale, Aug 28 2017 *)

Formula

a(n) = f(n+1, 0) with f(n, x) = if n=1 then A004086(x) else f(floor(n/2), 10*x + 3 + 4*(n mod 2)).