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A284971
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Numbers with digits 4 and 7 only.
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5
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4, 7, 44, 47, 74, 77, 444, 447, 474, 477, 744, 747, 774, 777, 4444, 4447, 4474, 4477, 4744, 4747, 4774, 4777, 7444, 7447, 7474, 7477, 7744, 7747, 7774, 7777, 44444, 44447, 44474, 44477, 44744, 44747, 44774, 44777, 47444, 47447, 47474, 47477, 47744, 47747
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listen;
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internal format)
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OFFSET
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1,1
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LINKS
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MATHEMATICA
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Flatten@ Table[FromDigits /@ Tuples[{4, 7}, n], {n, 5}] (* Giovanni Resta, Apr 08 2017 *)
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PROG
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(Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {4, 7}]
(PARI) is(n) = my(x=Set([4, 7]), y=Set([0, 1, 2, 3, 5, 6, 8, 9])); if(#setintersect(Set(digits(n)), x) > 0 && #setintersect(Set(digits(n)), y)==0, return(1)); 0 \\ Felix Fröhlich, Apr 08 2017
(Python)
def a(n):
b = bin(n+1)[3:]
return int("".join(b.replace("0", "4").replace("1", "7")))
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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