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A046413 Numbers n such that the repunit of length n (11...11, with n 1's) has exactly 2 prime factors. 11
3, 4, 5, 7, 11, 17, 47, 59, 71, 139, 211, 251, 311, 347, 457, 461 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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, Mar 11 2003

The Yousuke Koide reference now shows repunit of length 263 partially factored, no longer possible candidate for this sequence. - Ray Chandler, Sep 06 2005

The term 263 has 3 prime factors, 617 has one prime factor and a large composite. For terms between 1000 to 2000, other possible terms are 1117, 1213, 1259, 1291, 1373, 1447, 1607, 1637, 1663, 1669, 1759, 1823, 1949, 1987, 2063 & 2087. - Robert G. Wilson v, Apr 26 2010

All terms are either primes or squares of primes in A004023. In particular, the only composite below a million is 4. - Charles R Greathouse IV, Nov 21 2014

a(17) >= 509. The only confirmed term below 2500 is 701. As of July 2019, no factorization is known for the potential terms 509, 557, 647, 991, 1117, 1259, 1447, 1607, 1637, 1663, 1669, 1759, 1823, 1949, 1987, 2063, 2087, 2111, 2203, 2269, 2309, 2341, 2467, 2503, 2521, ... Unless the least prime factors of the respective composites have less than ~80 decimal digits and are thus accessible by massive ECM computations, there is no chance for an extension using current publicly available factorization methods. See links to factordb.com for the status of the factorization of the smallest unknown terms. - Hugo Pfoertner, Jul 30 2019

REFERENCES

Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60.

LINKS

Table of n, a(n) for n=1..16.

P. De Geest, Repunits prime factors

Makoto Kamada, Factorizations of 11...11 (Repunit).

Yousuke Kiode, Factorizations of Repunit Numbers.

Eric Weisstein's World of Mathematics, Repunit

Status of (10^509-1)/9 in factordb.com.

Status of (10^557-1)/9 in factordb.com.

Status of (10^647-1)/9 in factordb.com.

EXAMPLE

a(n)=7 so 1111111 = 239*4649.

CROSSREFS

Cf. A000042, A004022 (repunit primes), A046053, A102782.

Sequence in context: A095880 A076497 A137950 * A285224 A322991 A120635

Adjacent sequences:  A046410 A046411 A046412 * A046414 A046415 A046416

KEYWORD

nonn,base,more,hard

AUTHOR

Patrick De Geest, Jul 15 1998

EXTENSIONS

More terms from Rick L. Shepherd, Mar 11 2003

a(13)-a(16) from Robert G. Wilson v, Apr 26 2010

STATUS

approved

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Last modified October 18 12:18 EDT 2019. Contains 328160 sequences. (Running on oeis4.)