

A056804


Numbers n such that 7*10^n + 1 is prime.


6



1, 2, 3, 4, 5, 8, 9, 45, 136, 142, 158, 243, 923, 1235, 2196, 4650, 6119, 7324, 9543, 13494, 20310, 20360, 232920, 830865, 902708
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OFFSET

1,2


COMMENTS

7*10^902708+1 was the 84thlargest prime in the world as of Jun 28 2013. Also largest known "base10" form for a prime number. There are no other values of n in this sequence between 232920 and 902708 that yield a prime.  Edward A. Trice, Jul 03 2013
Apparently Edward A. Trice found another term in the gap mentioned above in May 2014.  Ray Chandler, Apr 29 2015


LINKS

Table of n, a(n) for n=1..25.
Makoto Kamada, Prime numbers of the form 700...001.
Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers.
Index entries for primes involving repunits.


FORMULA

a(n) = A101128(n) + 1.


MATHEMATICA

Do[ If[ PrimeQ[ 7*10^n + 1], Print[ n ]], {n, 0, 10000}]


PROG

(PARI) is(n)=ispseudoprime(7*10^n+1) \\ Charles R Greathouse IV, Feb 20 2017


CROSSREFS

Cf. A101128.
Sequence in context: A057911 A085266 A280431 * A305399 A101547 A047597
Adjacent sequences: A056801 A056802 A056803 * A056805 A056806 A056807


KEYWORD

more,nonn


AUTHOR

Robert G. Wilson v, Aug 22 2000


EXTENSIONS

a(15)a(20) from Robert G. Wilson v, Jan 19 2005
a(21)a(22) from Kamada data by Robert Price, Dec 14 2010
a(23) from Edward A. Trice, Jan 11 2013
a(25) (thought to be a(24) at the time) from Edward A. Trice, Jun 28 2013
a(24) from Edward A. Trice, inserted by Robert G. Wilson v, May 15 2014


STATUS

approved



