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A285011
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Numbers with digits 7 and 9 only.
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0
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7, 9, 77, 79, 97, 99, 777, 779, 797, 799, 977, 979, 997, 999, 7777, 7779, 7797, 7799, 7977, 7979, 7997, 7999, 9777, 9779, 9797, 9799, 9977, 9979, 9997, 9999, 77777, 77779, 77797, 77799, 77977, 77979, 77997, 77999, 79777, 79779, 79797, 79799, 79977, 79979
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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[{7, 9}, n], {n, 5}] (* Giovanni Resta, Apr 10 2017 *)
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PROG
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(Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {7, 9}]
(PARI) a(n, {p=[7, 9]})={my(v=binary(n+1)); fromdigits(vector(#v-1, i, p[2]*v[i+1]+p[1]*!v[i+1]))} \\ R. J. Cano, Apr 09 2017
(Python)
def a(n): return int(bin(n+1)[3:].replace('0', '7').replace('1', '9'))
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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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