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A101066
Indices of primes in sequence defined by A(0) = 81, A(n) = 10*A(n-1) + 31 for n > 0.
1
25, 35, 37, 59, 79, 91, 173, 485, 626, 998, 1613, 4381, 4897, 8441, 17261, 17801, 35426, 40742
OFFSET
1,1
COMMENTS
Numbers n such that (760*10^n - 31)/9 is prime.
Numbers n such that digit 8 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
Numbers corresponding to terms <= 626 are certified primes.
a(19) > 10^5. - Robert Price, Oct 20 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103079(n+1) - 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
EXAMPLE
844444444444444444444444441 is prime, hence 25 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(760*10^# - 31)/9] &] (* Robert Price, Oct 20 2015 *)
PROG
(PARI) a=81; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 1000, if(isprime((760*10^n-31)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004
EXTENSIONS
Three additional terms, corresponding to probable primes, from Ryan Propper, Jun 20 2005
One more term from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
Edited by T. D. Noe, Oct 30 2008
a(15)-a(18) from Kamada data by Ray Chandler, Apr 29 2015
STATUS
approved