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A060376
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If 10^n can be written as x*y where the digits of x and y are all nonzero, then let a(n) = smallest such x, otherwise a(n) = -1.
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1
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1, 2, 4, 8, 16, 32, 64, 128, -1, 512, -1, -1, -1, -1, -1, -1, -1, -1, 262144, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 8589934592, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| According to Ogilvy and Anderson, 10^33 is the highest known power of ten that can be expressed as the product of two zero-free factors. "If there is another one, it is greater than 10^5000." p. 89
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REFERENCES
| C. Stanley Ogilvy and John T. Anderson, Excursions in Number Theory, Oxford University Press, 1966, p. 89.
Rudolph Ondrejka, Nonzero factors of 10^n, Recreational Mathematics Magazine, no. 6 (1961), p. 59.
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EXAMPLE
| 10^2 = 4 * 25, so a(2) = 4.
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CROSSREFS
| Cf. A060391 (for values of y).
Sequence in context: A122189 A194630 A133024 * A047869 A016025 A036161
Adjacent sequences: A060373 A060374 A060375 * A060377 A060378 A060379
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KEYWORD
| sign,base
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AUTHOR
| Jason Earls (zevi_35711(AT)yahoo.com), Apr 02 2001
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