A093401 -id:A093401 - cp's OEIS frontend

A099410 Numbers k such that 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 5, 14, 176, 416, 2505, 2759, 7925, 9401, 10391, 12105, 19616, 261704, 264539

Offset: 1

Robert G. Wilson v, Oct 14 2004

nonn

Also numbers k such that (2*10^k + 61)/9 is prime.
Although perhaps a degenerate case, A002275 defines R(0)=0.  Thus zero belongs in this sequence since 2*0+7=7 is prime. - Robert Price, Oct 28 2014
a(15) > 10^5. - Robert Price, Oct 29 2014

Makoto Kamada, Prime numbers of the form 22...229.
Index entries for primes involving repunits

Cf. A002275, A056678, A093401.

[n: n in [0..500] | IsPrime((2*10^n+61) div 9)]; // Vincenzo Librandi, Oct 30 2014

Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

a(n) = A056678(n-1) + 1.

Added zero and adapted Mathematica program by Robert Price, Oct 28 2014
a(10)-a(14) from Kamada data by Robert Price, Oct 29 2014
a(15)-a(16) from Kamada data by Tyler Busby, May 03 2024

A056678 Numbers k such that 20*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615

Offset: 1

Robert G. Wilson v, Aug 10 2000

Also numbers k such that (2*10^(k+1)+61)/9 is prime.

a(14) > 10^5. - Robert Price, Nov 01 2014

Makoto Kamada, Prime numbers of the form 22...229.
Index entries for primes involving repunits

Cf. A002275, A093401 (corresponding primes), A099410.

Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

a(n) = A099410(n+1) - 1. - Robert Price, Nov 01 2014

Two more terms from Rick L. Shepherd, Mar 30 2004
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(11)-a(13) derived from A099410 by Robert Price, Nov 01 2014

cp's OEIS Frontend

A099410 Numbers k such that 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

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