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A101841
Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) + 61 for n > 0.
1
0, 4, 70, 111, 114, 172, 262, 309, 357, 10498, 22924, 37956, 254194
OFFSET
1,2
COMMENTS
Numbers n such that (340*10^n - 61)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.
Numbers corresponding to terms <= 357 are certified primes.
a(14) >= 4*10^5. - Jason H Parker, Jun 15 2019
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A102976(n) - 1. [adapted by Georg Fischer, Jan 04 2021]
EXAMPLE
377771 is prime, hence 4 is a term.
MATHEMATICA
Select[Range[0, 300], PrimeQ[(340*10^# - 61)/9] &] (* Robert Price, May 08 2015 *)
PROG
(PARI) a=31; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+61)
(PARI) for(n=0, 1500, if(isprime((340*10^n-61)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
a(10)-a(12) from Kamada data by Ray Chandler, May 01 2015
a(13) from Jason H Parker, Jun 15 2019
STATUS
approved