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A101969
Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 71 for n > 0.
1
1, 11, 26, 43, 79, 118, 130, 274, 314, 875, 1306, 10894, 17104, 60716
OFFSET
1,2
COMMENTS
Numbers n such that (260*10^n - 71)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
Numbers corresponding to terms <= 875 are certified primes.
a(15) > 10^5. - Robert Price, Mar 16 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A102960(n) - 1.
EXAMPLE
281 is prime, hence 1 is a term.
MATHEMATICA
Select[Range[1000], PrimeQ[(260*10^# - 71)/9] &] (*Robert Price, Mar 17 2015*)
PROG
(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1500, if(isprime((260*10^n-71)/9), print1(n, ", ")))
(Magma) [n: n in [0..350] | IsPrime((260*10^n - 71) div 9)]; // Vincenzo Librandi, Mar 17 2015
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
EXTENSIONS
a(12)-a(14) derived from A102960 by Robert Price, Mar 16 2015
STATUS
approved