A096846
Numbers n for which 8*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
Original entry on oeis.org
1, 3, 4, 6, 9, 12, 72, 118, 124, 190, 244, 304, 357, 1422, 2691, 5538, 7581, 21906, 32176, 44358, 120552, 137073, 152260
Offset: 1
n=6: a(4)=888887 which is prime.
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Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 1], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 15 2004; corrected by Derek Orr, Sep 06 2014 *)
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for(n=1,10^4,if(ispseudoprime(8*(10^n-1)/9-1),print1(n,", "))) \\ Derek Orr, Sep 06 2014
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(18)-a(20) discovered and reported to Makoto Kamada by Erik Branger; added to OEIS by
Robert Price, May 20 2014
a(21)-a(23) from Kamada data by
Tyler Busby, Apr 23 2024
A173812
a(n) = (8*10^n - 17)/9 for n > 0.
Original entry on oeis.org
7, 87, 887, 8887, 88887, 888887, 8888887, 88888887, 888888887, 8888888887, 88888888887, 888888888887, 8888888888887, 88888888888887, 888888888888887, 8888888888888887, 88888888888888887, 888888888888888887, 8888888888888888887, 88888888888888888887, 888888888888888888887
Offset: 1
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[(8*10^n-17)/9: n in [1..20]]; // Vincenzo Librandi, Jul 05 2012
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CoefficientList[Series[(7+10*x)/((1-x)*(1-10*x)),{x,0,30}],x] (* Vincenzo Librandi, Jul 05 2012 *)
Table[FromDigits[PadLeft[{7},n,8]],{n,20}] (* Harvey P. Dale, Jun 22 2013 *)
A056695
Numbers k such that 80*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
Original entry on oeis.org
0, 2, 3, 5, 8, 11, 71, 117, 123, 189, 243, 303, 356, 1421, 2690, 5537, 7580, 21905, 32175, 44357
Offset: 1
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Do[ If[ PrimeQ[80*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
A096843
Primes of form repdigit - 1. Primes whose sum of divisors is a decimal repdigit.
Original entry on oeis.org
2, 3, 5, 7, 43, 443, 887, 2221, 8887, 444443, 888887, 444444443, 888888887, 444444444443, 888888888887, 222222222222222221, 444444444444444444444444444443, 44444444444444444444444444444443
Offset: 1
Showing 1-4 of 4 results.
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