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A189555
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Numbers n such that x' = n has two solutions, where x' is the arithmetic derivative (A003415) of x.
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2
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10, 12, 14, 18, 20, 21, 28, 31, 38, 39, 45, 55, 61, 71, 81, 87, 101, 103, 111, 119, 123, 129, 131, 147, 183, 185, 199, 211, 213, 215, 241, 243, 255, 269, 291, 297, 299, 327, 339, 343, 351, 355, 359, 361, 363, 381, 395, 399, 401, 411, 421, 433, 439, 471, 493
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OFFSET
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1,1
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COMMENTS
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Ufnarovski and Ahlander conjecture that this sequence, and any such sequence that has numbers n such that x' = n has k solutions, is infinite. See A098700 and A189481 for the k=0 and 1 cases. It appears that the only even terms here are 10, 12, 14, 18, 20, 28, and 38. The prime terms are in A189556.
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REFERENCES
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LINKS
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FORMULA
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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