login
A099412
0 together with numbers k such that 4*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
0, 2, 4, 11, 28, 55, 94, 475, 2080, 4835, 5845, 12338, 20194, 49529, 56989, 71546, 77309, 284330
OFFSET
1,2
COMMENTS
Also numbers k such that abs((4*10^k - 31))/9 is prime.
a(18) > 10^5. - Robert Price Sep 06 2014
FORMULA
a(n) = A056681(n-1) + 1.
MATHEMATICA
Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 3], Print[n]], {n, 0, 7000}]
PROG
(PARI)
for(n=0, 10^4, if(ispseudoprime(abs(4*(10^n-1)/9-3)), print1(n, ", "))) \\ Derek Orr, Sep 06 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
a(12)-a(17) from Kamada data by Robert Price, Sep 06 2014
a(18) from Kamada data by Tyler Busby, Apr 30 2024
STATUS
approved