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A084428 Numbers n such that n 7's followed by n is prime. 7
1, 23, 209, 1171, 6239, 7543 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(7) > 15000. - Derek Orr, Jul 29 2014

a(7) > 10^5. - Robert Price, Apr 05 2019

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

23 is in the sequence because 7777777777777777777777723(23 7's followed by 23) is prime.

MATHEMATICA

Do[ If[GCD[n, 30] == 1 && PrimeQ[ FromDigits[ Join[ Table[7, {n}], IntegerDigits[n]]]], Print[n]], {n, 1, 6500}]

PROG

(PARI)

for(n=1, 15000, p=""; for(k=1, n, p=concat(p, Str(7))); p=concat(p, Str(n)); if(ispseudoprime(eval(p)), print1(n, ", "))) \\ Derek Orr, Jul 29 2014

CROSSREFS

Cf. A070746, A068817.

Sequence in context: A022683 A042020 A263521 * A322287 A327918 A327919

Adjacent sequences:  A084425 A084426 A084427 * A084429 A084430 A084431

KEYWORD

nonn,base,more,hard

AUTHOR

Farideh Firoozbakht, Jun 27 2003

EXTENSIONS

a(6) from Derek Orr, Jul 29 2014

STATUS

approved

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Last modified May 12 07:28 EDT 2021. Contains 343821 sequences. (Running on oeis4.)