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A101072 Indices of primes in sequence defined by A(0) = 87, A(n) = 10*A(n-1) - 13 for n > 0. 1
1, 7, 1055, 19834, 20017, 36463, 81605 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that (770*10^n + 13)/9 is prime.

Numbers n such that digit 8 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.

Some of the larger entries may only correspond to probable primes.

a(8) > 10^5. - Robert Price, Oct 25 2015

REFERENCES

Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

LINKS

Table of n, a(n) for n=1..7.

Makoto Kamada, Prime numbers of the form 855...557.

Index entries for primes involving repunits.

FORMULA

a(n) = A103085(n) - 1.

EXAMPLE

855555557 is prime, hence 7 is a term.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(770*10^# + 13)/9] &] (* Robert Price, Oct 25 2015 *)

PROG

(PARI) a=87; for(n=0, 2300, if(isprime(a), print1(n, ", ")); a=10*a-13)

(PARI) for(n=0, 2300, if(isprime((770*10^n+13)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A103085.

Sequence in context: A250914 A004807 A139781 * A203583 A259251 A259253

Adjacent sequences:  A101069 A101070 A101071 * A101073 A101074 A101075

KEYWORD

nonn,hard,more

AUTHOR

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004

EXTENSIONS

a(4)-a(6) from Kamada data by Ray Chandler, Apr 29 2015

a(7) from Robert Price, Oct 25 2015

STATUS

approved

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Last modified September 23 07:41 EDT 2021. Contains 347609 sequences. (Running on oeis4.)