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A056260
Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 3 for n > 0. Numbers n such that (690*10^n + 3)/9 is prime.
2
3, 5, 53, 95, 453, 573, 3383, 11439, 12623, 19445, 35459, 81213, 95325
OFFSET
1,1
COMMENTS
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 7 is prime.
Numbers corresponding to terms <= 3383 are certified primes.
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
FORMULA
a(n) = A082714(n) - 2.
EXAMPLE
76667 is prime, hence 3 is a term.
MATHEMATICA
Select[Range[3500], PrimeQ[(690 10^# + 3) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)
PROG
(PARI) a=77; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-3)
(PARI) for(n=0, 1000, if(isprime((690*10^n+3)/9), print1(n, ", ")))
(Magma) [n: n in [0..300] | IsPrime((690*10^n+3) div 9)]; // Vincenzo Librandi, Nov 03 2014
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 18 2000
EXTENSIONS
Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, Apr 29 2007, Jun 15 2007
a(7)-a(11) from Robert G. Wilson v, May 02 2007
Two more terms added from PDP Table, a link added and comments section updated by Patrick De Geest, Nov 02 2014
Edited by Ray Chandler, Nov 05 2014
STATUS
approved