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%I #7 Aug 28 2017 15:16:16
%S 3,7,33,37,73,77,333,337,373,377,733,737,773,777,3333,3337,3373,3377,
%T 3733,3737,3773,3777,7333,7337,7373,7377,7733,7737,7773,7777,33333,
%U 33337,33373,33377,33733,33737,33773,33777,37333,37337,37373,37377
%N Numbers containing only digits 3 or 7 in decimal representation.
%C See A020463 for primes.
%H Reinhard Zumkeller, <a href="/A143967/b143967.txt">Table of n, a(n) for n = 1..10000</a>
%F 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)).
%t Table[FromDigits/@Tuples[{3,7},n],{n,5}]//Flatten (* _Harvey P. Dale_, Aug 28 2017 *)
%o (Haskell)
%o a143967 = f 0 . (+ 1) where
%o f y 1 = a004086 y
%o f y x = f (10 * y + 3 + 4 * r) x' where (x', r) = divMod x 2
%o -- _Reinhard Zumkeller_, Mar 18 2015
%Y Cf. A004086, A020463, A032810.
%K base,nonn
%O 1,1
%A _Reinhard Zumkeller_, Sep 06 2008
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