

A056708


Numbers n such that 4*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.


1



1, 4, 13, 25, 36, 357, 373, 1041, 1089, 1093, 1297, 8274, 10732, 15972, 18114, 21823, 34519, 36096, 75498
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OFFSET

1,2


COMMENTS

Also numbers n such that (43*10^n7)/9 is prime.
a(20) > 10^5.  Robert Price, Dec 22 2014


LINKS

Table of n, a(n) for n=1..19.
Makoto Kamada, Prime numbers of the form 477...77.
Index entries for primes involving repunits


MATHEMATICA

Do[ If[ PrimeQ[ 4*10^n + 7*(10^n1)/9], Print[n]], {n, 0, 6000}]


CROSSREFS

Cf. A002275, A093940.
Sequence in context: A272734 A273414 A154820 * A307271 A092484 A091823
Adjacent sequences: A056705 A056706 A056707 * A056709 A056710 A056711


KEYWORD

hard,nonn


AUTHOR

Robert G. Wilson v, Aug 10 2000


EXTENSIONS

8274 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(13)a(19) from Robert Price, Dec 22 2014


STATUS

approved



