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A143967
Numbers containing only digits 3 or 7 in decimal representation.
7
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
1,1
COMMENTS
See A020463 for primes.
LINKS
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)).
MATHEMATICA
Table[FromDigits/@Tuples[{3, 7}, n], {n, 5}]//Flatten (* Harvey P. Dale, Aug 28 2017 *)
PROG
(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
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Reinhard Zumkeller, Sep 06 2008
STATUS
approved