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A101578
Indices of primes in sequence defined by A(0) = 51, A(n) = 10*A(n-1) + 31 for n > 0.
1
1, 2, 85, 133, 184, 343, 395, 475, 833, 1798, 2146, 2417, 5215, 5881, 6215, 13393, 19745, 66484
OFFSET
1,2
COMMENTS
Numbers n such that (490*10^n - 31)/9 is prime.
Numbers n such that digit 5 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
a(19) > 10^5. - Robert Price, Jul 16 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103013(n+1) - 1.
EXAMPLE
5441 is prime, hence 2 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(490*10^#-31)/9]&] (* Robert Price, Jul 16 2015 *)
PROG
(PARI) a=51; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 1000, if(isprime((490*10^n-31)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 2004
EXTENSIONS
3 more terms from Ryan Propper, Jun 16 2005
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Apr 28 2007
a(16)-a(17) by Ray Chandler, Apr 24 2015
a(18) from Robert Price, Jul 16 2015
STATUS
approved