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A102946 Numbers n such that 2*10^n - 7 is prime. 2
1, 2, 3, 4, 6, 16, 21, 28, 48, 82, 122, 130, 282, 304, 4602, 12984, 13614, 42762, 90597, 109928, 158242 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that 10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

a(20) > 10^5. - Robert Price, Nov 16 2014

a(22) > 2*10^5. - Robert Price, Oct 25 2015

LINKS

Table of n, a(n) for n=1..21.

Makoto Kamada, Prime numbers of the form 199...993.

Index entries for primes involving repunits.

FORMULA

a(n) = A102033(n) + 1.

MAPLE

select(n -> isprime(2*10^n-7), [$0..10^4]); # Robert Israel, Nov 16 2014

MATHEMATICA

Do[ If[ PrimeQ[2*10^n - 7], Print[n]], {n, 0, 10000}]

Select[Range[1000], PrimeQ[(2 10^# - 7)] &] (* Vincenzo Librandi, Nov 17 2014 *)

PROG

(MAGMA) [n: n in [1..500] | IsPrime(2*10^n-7)]; // Vincenzo Librandi, Nov 17 2014

(PARI) is(n)=ispseudoprime(2*10^n-7) \\ Charles R Greathouse IV, Jun 06 2017

CROSSREFS

Cf. A002275, A102033.

Sequence in context: A280592 A002087 A049288 * A026094 A285191 A271360

Adjacent sequences:  A102943 A102944 A102945 * A102947 A102948 A102949

KEYWORD

more,nonn

AUTHOR

Robert G. Wilson v, Dec 16 2004

EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

Addition of a(18)-a(19) from Kamada data by Robert Price, Dec 10 2010

a(20)-a(21) from Robert Price, Oct 25 2015

STATUS

approved

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Last modified December 12 20:12 EST 2019. Contains 329961 sequences. (Running on oeis4.)