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 A056259 Indices of primes in sequence defined by A(0) = 77, A(n) = 10*A(n-1) - 13 for n > 0. 1
 1, 3, 9, 19, 21, 57, 73, 81, 207, 349, 421, 3811, 3981, 20923, 23785, 38851, 56041, 68503, 74433 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (680*10^n + 13)/9 is prime. Numbers n such that digit 7 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime. Numbers corresponding to terms <= 3981 are certified primes. REFERENCES Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. LINKS Patrick De Geest, PDP Reference Table - 757. Makoto Kamada, Prime numbers of the form 755...557. FORMULA a(n) = A082713(n) - 2. EXAMPLE 75557 is prime, hence 3 is a term. MATHEMATICA Select[Range[0, 2000], PrimeQ[(680 10^# + 13) / 9] &] (* Vincenzo Librandi, Nov 03 2014 PROG (PARI) a=77; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-13) (PARI) for(n=0, 1000, if(isprime((680*10^n+13)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A082713. Sequence in context: A043097 A084670 A002091 * A056682 A032670 A068677 Adjacent sequences:  A056256 A056257 A056258 * A056260 A056261 A056262 KEYWORD nonn,hard,more AUTHOR Robert G. Wilson v, Aug 18 2000 EXTENSIONS Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004 Edited by N. J. A. Sloane at the suggestion of Andrew Plewe, Jun 05 2007 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 Three more terms added from Table, a link added, and comments section updated by Patrick De Geest, Nov 02 2014 Edited by Ray Chandler, Nov 05 2014 STATUS approved

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Last modified September 23 23:21 EDT 2018. Contains 315306 sequences. (Running on oeis4.)