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A101540
Indices of primes in sequence defined by A(0) = 69, A(n) = 10*A(n-1) - 1 for n > 0.
1
4, 56, 500, 514, 626, 640, 724, 53110, 65330, 109672
OFFSET
1,1
COMMENTS
Numbers n such that (620*10^n + 1)/9 is prime.
Numbers n such that digit 6 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.
Numbers corresponding to terms <= 724 are certified primes.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103047(n) - 1.
EXAMPLE
688889 is prime, hence 4 is a term.
PROG
(PARI) a=69; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-1)
(PARI) for(n=0, 1000, if(isprime((620*10^n+1)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004
EXTENSIONS
a(8)-a(10) from Kamada data by Ray Chandler, Apr 30 2015
STATUS
approved