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A101847
Indices of primes in sequence defined by A(0) = 39, A(n) = 10*A(n-1) - 1 for n > 0.
1
1, 2, 5, 16, 19, 44, 56, 77, 118, 136, 508, 709, 1126, 1517, 1760, 3085, 3146, 9925, 19853, 27841, 46798, 49681, 68717, 100541, 106648, 191072
OFFSET
1,2
COMMENTS
Numbers n such that (350*10^n + 1)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.
Numbers corresponding to terms <= 709 are certified primes.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A102980(n) - 1.
EXAMPLE
3889 is prime, hence 2 is a term.
MATHEMATICA
Select[Range[0, 1000], PrimeQ[(350*10^# + 1)/9]&]
PROG
(PARI) a=39; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-1)
(PARI) for(n=0, 1500, if(isprime((350*10^n+1)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(19)-a(26) derived from A102980 by Robert Price, Mar 17 2015
STATUS
approved