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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
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.
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
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