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A256606 Numbers k such that 3*R_k + 40 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
1, 2, 3, 4, 7, 8, 33, 38, 157, 252, 359, 365, 567, 876, 3108, 5780, 12987, 14984, 22287, 31574, 37473, 40984, 49806, 51364, 62451 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (10^k + 119)/3 is prime.

Terms from Kamada data.  Note Kamada does not recognize k=1 as 43 is a degenerate case of form AAA..ABA.

a(26) > 2.5*10^5.

LINKS

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Prime numbers of the form 33...3373.

Index entries for primes involving repunits.

EXAMPLE

For k=3, 3*R_3 + 40 = 333 + 40 = 373 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(10^# + 119)/3] &]

PROG

(PARI) is(n)=ispseudoprime(10^n\3+40) \\ Charles R Greathouse IV, Apr 10 2015

(MAGMA) [n: n in [0..400] | IsPrime((10^n+119) div 3)]; // Vincenzo Librandi, Apr 11 2015

CROSSREFS

Cf. A002275.

Sequence in context: A134459 A225211 A159554 * A101128 A182802 A172024

Adjacent sequences:  A256603 A256604 A256605 * A256607 A256608 A256609

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 10 2015

STATUS

approved

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Last modified April 4 21:43 EDT 2020. Contains 333238 sequences. (Running on oeis4.)