|
Search: id:A046413
|
|
|
| A046413 |
|
Numbers n such that the repunit of length n (11...11, with n 1's) has exactly 2 prime factors. |
|
+0
9
|
|
| 3, 4, 5, 7, 11, 17, 47, 59, 71, 139, 211, 251
(list; graph; listen)
|
|
|
OFFSET
|
1,1
|
|
|
COMMENT
|
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
|
|
REFERENCES
|
Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60.
|
|
LINKS
|
P. De Geest, Repunits prime factors
Yousuke KOIDE, Factorizations of Repunit Numbers.
Eric Weisstein's World of Mathematics, Repunit
|
|
EXAMPLE
|
a(n)=7 so 1111111 = 239*4649.
|
|
CROSSREFS
|
Cf. A000042, A004022 (the actual primes), A046053, A102782.
Adjacent sequences: A046410 A046411 A046412 this_sequence A046414 A046415 A046416
Sequence in context: A139455 A095880 A076497 this_sequence A120635 A113533 A023713
|
|
KEYWORD
|
nonn,base
|
|
AUTHOR
|
Patrick De Geest (pdg(AT)worldofnumbers.com), Jul 15 1998.
|
|
EXTENSIONS
|
More terms from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 11 2003
|
|
|
Search completed in 0.002 seconds
|
