login
A102987
Numbers n such that 4*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1
0, 3, 2043, 5967, 39855
OFFSET
1,2
COMMENTS
Also numbers n such that (38*10^n+61)/9 is prime.
a(6) > 10^5. - Robert Price, May 08 2015
FORMULA
a(n) = A101722(n-1) + 1.
MATHEMATICA
Do[ If[ PrimeQ[(38*10^n + 61)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A307928 A193149 A024047 * A004814 A203497 A145700
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Jan 17 2005
EXTENSIONS
Addition of a(5) from Kamada data by Robert Price, Dec 13 2010
STATUS
approved