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A101140
Indices of primes in sequence defined by A(0) = 73, A(n) = 10*A(n-1) + 13 for n > 0.
1
0, 1, 18, 30, 105, 26193, 39972
OFFSET
1,3
COMMENTS
Numbers n such that (670*10^n - 13)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 4 followed by digit 3 is prime.
Numbers corresponding to terms <= 105 are certified primes.
The next term, if it exists, is bigger than 4000. - Stefan Steinerberger, Feb 04 2006
a(8) > 10^5. - Robert Price, Sep 25 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A103058(n) - 1.
EXAMPLE
73 is prime, hence 0 is a term.
MATHEMATICA
For[n=0, n < 4000, n++, If[PrimeQ[(670*10^n - 13)/9], Print[n]]] (Steinerberger)
PROG
(PARI) a=73; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a+13)
(PARI) for(n=0, 1000, if(isprime((670*10^n-13)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
EXTENSIONS
a(6) from Kamada data by Ray Chandler, Apr 30 2015
a(7) from Robert Price, Sep 25 2015
STATUS
approved