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A102021
Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 31 for n > 0.
1
0, 2, 3, 6, 11, 50, 86, 122, 197, 201, 536, 830, 1322, 2507, 2630, 5567, 6896, 7523, 9938, 10673, 11616, 13256, 14370, 20175, 36554, 36873, 40130, 43124, 58728, 97043
OFFSET
1,2
COMMENTS
Numbers n such that (140*10^n + 31)/9 is prime.
Numbers n such that digit 1 followed by n >= 0 occurrences of digit 5 followed by digit 9 is prime.
Numbers corresponding to terms <= 830 are certified primes.
a(31) > 10^5. Robert Price, Feb 03 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A102938(n) - 1.
EXAMPLE
1559 is prime, hence 2 is a term.
PROG
(PARI) a=19; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-31)
(PARI) for(n=0, 1500, if(isprime((140*10^n+31)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(20)-a(30) from Robert Price, Feb 03 2015
STATUS
approved