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A256934 Numbers k such that R_(k+2) + 8*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
0, 1, 12, 16, 33, 37, 42, 6643, 35157, 63202, 125292, 200746 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also, numbers k such that (172*10^k-1)/9  is prime.

Terms from Kamada.

a(13) > 250000.

LINKS

Table of n, a(n) for n=1..12.

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

Makoto Kamada, Prime numbers of the form 1911...11.

Index entries for primes involving repunits.

EXAMPLE

For k=1, R_3 + 8*10^1 = 111 + 80 = 191 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(172*10^#-1)/9 ] &]

PROG

(MAGMA) [n: n in [0..300] | IsPrime((172*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015

(PARI) for(n=0, 300, if(isprime((172*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

CROSSREFS

Cf. A002275.

Sequence in context: A109240 A020740 A086980 * A058203 A323978 A219390

Adjacent sequences:  A256931 A256932 A256933 * A256935 A256936 A256937

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 13 2015

STATUS

approved

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Last modified June 2 06:39 EDT 2020. Contains 334767 sequences. (Running on oeis4.)