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Repunits and their prime factors
Normal Smarandache Concatenated Numbers Reversed Smarandache Concatenated Numbers Circular Primes

In the table below you'll find all the prime factors of the first 100 repunits.
The first not yet factored repunit seems to be R353 with a composite of 328 digits.
Makoto Kamada, Factorizations of 11...11 (repunits).

Some repunits prime factor lists on the web

Listing 1 - Repunit100 - date.pdf from Koide Yousuke
Listing 2 - Factors of 10^n – 1, n odd, n ⩽ 999 from Torbjörn Granlund
Listing 3 - Factorizations of 10^n – 1, n odd, n < 400 by Sam Wagstaff.
Listing 4 - Cunningham Book, Third Edition, Online version

Some interesting repunits websources

Factorizations of Repunit Numbers - Repunit_factors.txt, upto n=8580 ! (archived version) by Koide Yousuke from Japan.
Repunit by Eric W. Weisstein
The Cunningham Project by Sam Wagstaff
OEIS Sequence A000042 - Unary representation of natural numbers.
OEIS Sequence A002275 - Repunits: (10^n – 1)/9.
OEIS Sequence A004023 - Indices of prime "repunits": numbers n such that 11...111 = (10^n – 1)/9 is prime.
OEIS Sequence A067063 - Smallest prime factor of repunit(n) = (10^n – 1)/9.
OEIS Sequence A003020 - Largest factor of the "repunit" number 11...1.

Some repunits archived 'Yahoo.Groups.Primeform' messages about R(86453)

Message 1128 Message 1132 Message 1133 Message 1134 Message 1139 Message 1140 Message 1141 Message 1142 Message 1143 Message 1144

Some 'NMBRTHRY@LISTSERV.NODAK.EDU' messages about repunits

New PRP Repunit R(270343) from 15 Jul 2007 by Maksym Voznyy & Anton Budnyy
New Repunit R(109297) from 3 Apr 2007 by Harvey Dubner
R86453 is a New Probable Prime Repunit from 26 Oct 2000 by Lew Baxter
New prp Repunit, R(49081) from 9 Sep 1999 by Harvey Dubner

[ April 20, 2022 ]
Message forwarded to me by Xinyao (Richard) Chen (email).

The repunit R(49081) is now proven prime by Paul Underwood [ March 21, 2022 ], see https://www.mersenneforum.org/showpost.php?p=602219&postcount=35

Also, the repunit R(323) was factored, see http://pastebin.com/k9ahZHbp, currently the first not yet factored repunit is R(353), with a composite of 328 digits.

[ May 8, 2021 ]
Serge Batalov and Ryan Propper announce a [ New Probable Prime Repunit, R(8177207) ]

References

Serge Batalov "New repunit (PRP) prime found, 8177207 decimal digits, a PRP record".
https://mersenneforum.org/showthread.php?p=578120#

Sloane's OEIS sequence A004023 - Indices of prime "repunits": numbers n such that 11...111 = (10^n – 1)/9 is prime.

[ April 20, 2021 ]
Serge Batalov and Ryan Propper announce a [ New Probable Prime Repunit, R(5794777) ]

see also my table at webpage about circular primes.

References

Serge Batalov "New repunit (PRP) prime found, 5794777 decimal digits, a PRP record".
https://www.mersenneforum.org/showthread.php?p=576330#
https://groups.io/g/primenumberstheory/message/94

Sloane's OEIS sequence A004023 - Indices of prime "repunits": numbers n such that 11...111 = (10^n – 1)/9 is prime.

[ July 11, 2007 ]
Maksym Voznyy and Anton Budnyy (email) announce a New Probable Prime Repunit, R(270343)

see also my table at webpage about circular primes.

References

Voznyy, M. "New PRP Repunit R(270343)" NMBRTHRY@LISTSERV.NODAK.EDU mailing list. 15 Jul 2007.
https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;1a44c5be.0707&S=

[ April 3, 2007 ]
Harvey Dubner announces a New Probable Prime Repunit, R(109297)

[ January 9, 2006 ]
Evgenii Meerson (email) from Jerusalem (Israel) recently wrote a short note where
new results about the repunits are presented.
See 'Repunits and Harmony.pdf' in his site at https://web.archive.org/web/20190710052947/http://evgenii-meerson-pr.tripod.com/.
Evgenii hopes that his results may be useful for everyone showing interest in the
repunit properties.

[ May 20, 2001 ] Last update [ June 18, 2022 ]

Hans Havermann (email) provides now a list with prime factors upto repunit R(352).
Hans' website is at Factorization of Repunits, n < 353

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