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A101848
Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) + 81 for n > 0.
1
0, 16, 18, 28, 42, 118, 172, 948, 1608, 5578, 19678, 34146, 43492, 97798
OFFSET
1,2
COMMENTS
Numbers n such that (360*10^n - 81)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
Numbers corresponding to terms <= 948 are certified primes.
a(15) > 10^5. - Robert Price, Mar 17 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A101396(n) - 1.
EXAMPLE
31 is prime, hence 0 is a term.
MATHEMATICA
Select[Range[0, 1000], PrimeQ[(360*10^# - 81)/9] &] (* Robert Price, Mar 17 2015 *)
PROG
(PARI) a=31; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81)
(PARI) for(n=0, 1500, if(isprime((360*10^n-81)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(12)-a(14) derived from A101396 by Robert Price, Mar 17 2015
STATUS
approved