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A056678
Numbers k such that 20*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615
OFFSET
1,2
COMMENTS
Also numbers k such that (2*10^(k+1)+61)/9 is prime.
a(14) > 10^5. - Robert Price, Nov 01 2014
FORMULA
a(n) = A099410(n+1) - 1. - Robert Price, Nov 01 2014
MATHEMATICA
Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
CROSSREFS
Cf. A002275, A093401 (corresponding primes), A099410.
Sequence in context: A091957 A327443 A376062 * A218655 A369998 A369606
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
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
STATUS
approved