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A101150
Indices of primes in sequence defined by A(0) = 79, A(n) = 10*A(n-1) - 21 for n > 0.
1
0, 1, 2, 5, 7, 17, 18, 20, 31, 47, 64, 71, 98, 119, 200, 307, 716, 967, 1343, 2131, 3566, 3967, 8326, 10597, 12464, 15874, 18894, 28610, 29417, 83692
OFFSET
1,3
COMMENTS
Numbers n such that (690*10^n + 21)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.
Numbers corresponding to terms <= 967 are certified primes.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103065(n) - 1.
EXAMPLE
7669 is prime, hence 2 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(690*10^# + 21)/9] &] (* Robert Price, Oct 08 2015 *)
PROG
(PARI) a=79; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-21)
(PARI) for(n=0, 1500, if(isprime((690*10^n+21)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(28)-a(29) from Kamada data by Ray Chandler, Apr 30 2015
a(30) from Robert Price, Oct 08 2015
STATUS
approved