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A256426 Numbers k such that 7*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
2, 3, 5, 17, 44, 47, 78, 504, 533, 768, 915, 1155, 1313, 7128, 8088, 15717, 33074 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (7*10^k + 173)/9 is prime.

Terms from Kamada.

a(18) > 33074.

LINKS

Table of n, a(n) for n=1..17.

Makoto Kamada, Near-repdigit numbers of the form AA...AABA.

Makoto Kamada, Prime numbers of the form 77...7797.

Index entries for primes involving repunits.

EXAMPLE

For k=3, 7*R_11 + 20 = 777 + 20 = 797 which is prime.

MATHEMATICA

Select[Range[0, 30000], PrimeQ[(7*10^# + 173)/9] &]

PROG

(MAGMA) [n: n in [0..400] | IsPrime((7*10^n + 173) div 9)]; // Vincenzo Librandi, Apr 13 2015

CROSSREFS

Cf. A002275.

Sequence in context: A211972 A076706 A243441 * A019350 A235630 A235636

Adjacent sequences:  A256423 A256424 A256425 * A256427 A256428 A256429

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 12 2015

STATUS

approved

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Last modified March 20 13:18 EDT 2019. Contains 321345 sequences. (Running on oeis4.)