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
Keywords
Links
Programs
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Magma
[n: n in [0..500] | IsPrime((2*10^n+61) div 9)]; // Vincenzo Librandi, Oct 30 2014
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Mathematica
Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
Formula
a(n) = A056678(n-1) + 1.
Extensions
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
Comments