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Search: id:A110605
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| A110605 |
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Numbers n whose base 7 representations, interpreted as base 10 integers, are semiprimes. |
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+0
7
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| 4, 6, 7, 11, 12, 15, 16, 19, 20, 24, 25, 26, 34, 36, 40, 44, 47, 55, 57, 61, 64, 65, 66, 73, 74, 78, 79, 80, 82, 83, 89, 92, 97, 99, 100, 101, 103, 104, 108, 109, 110, 113, 118, 124, 136, 137, 142, 145, 148, 149, 150, 152, 158, 162, 164, 167, 172, 173, 176, 181, 187
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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A108873 is the equivalent using base 3. A110602 is the equivalent using base 4. A110603 is the equivalent using base 5. A110604 is the equivalent using base 6. A110606 is the equivalent using base 8. A110607 is the equivalent using base 9.
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EXAMPLE
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a(1) = 4 because 4 (base 7) = 4 (base 10) = 2 * 2, a semiprime (A001358).
a(2) = 6 because 6 (base 7) = 6 (base 10) = 2 * 3
a(3) = 7 because 7 (base 7) = 10, and 10 (base 10) = 2 * 5.
a(4) = 11 because 11 (base 7) = 14, and 14 (base 10) = 2 * 7.
a(5) = 12 because 12 (base 7) = 15, and 15 (base 10) = 3 * 5.
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MATHEMATICA
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Select[Range[187], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 7]]] == 2 &] (*Chandler*)
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CROSSREFS
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Cf. A001358, A007093, A108873, A110602, A110603, A110604, A110606, A110607.
Sequence in context: A023635 A103057 A039590 this_sequence A102140 A047289 A022436
Adjacent sequences: A110602 A110603 A110604 this_sequence A110606 A110607 A110608
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KEYWORD
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base,easy,nonn
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AUTHOR
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Jonathan Vos Post (jvospost2(AT)yahoo.com), Jul 30 2005
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EXTENSIONS
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Extended by Ray Chandler (rayjchandler(AT)sbcglobal.net), Aug 05 2005
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