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A056720
Numbers k such that 7*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
1
0, 1, 2, 3, 5, 53, 56, 343, 908, 1079, 2204, 2379, 9134, 9371, 9728, 11039, 11247, 13275, 16781, 20138, 75368, 87863
OFFSET
1,3
COMMENTS
Also numbers k such that (22*10^k - 1)/3 is prime.
a(23) > 10^5. - Robert Price, Dec 26 2014
MATHEMATICA
Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A062608 A041791 A322947 * A100850 A309607 A060085
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(16)-a(22) from Robert Price, Dec 26 2014
STATUS
approved