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Search: id:A046413
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| A046413 |
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Numbers n such that the repunit of length n (11...11, with n 1's) has exactly 2 prime factors. |
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+0
9
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| 3, 4, 5, 7, 11, 17, 47, 59, 71, 139, 211, 251
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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347, 457, 461 and 701 are also terms. The only other possible terms up to 1000 are 263, 311, 509, 557, 617, 647 and 991; repunits of these lengths are known to be composite but the linked sources do not provide their factors. - Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 11 2003
The Yousuke Koide reference now shows repunit of length 263 partially factored, no longer possible candidate for this sequence. - Ray Chandler (rayjchandler(AT)sbcglobal.net), Sep 06 2005
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REFERENCES
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Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60.
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LINKS
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P. De Geest, Repunits prime factors
Yousuke KOIDE, Factorizations of Repunit Numbers.
Eric Weisstein's World of Mathematics, Repunit
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EXAMPLE
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a(n)=7 so 1111111 = 239*4649.
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CROSSREFS
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Cf. A000042, A004022 (the actual primes), A046053, A102782.
Sequence in context: A095880 A076497 A137950 this_sequence A120635 A113533 A023713
Adjacent sequences: A046410 A046411 A046412 this_sequence A046414 A046415 A046416
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KEYWORD
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nonn,base
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AUTHOR
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Patrick De Geest (pdg(AT)worldofnumbers.com), Jul 15 1998.
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EXTENSIONS
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More terms from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 11 2003
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