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 A056254 Indices of primes in sequence defined by A(0) = 33, A(n) = 10*A(n-1) + 23 for n > 0. 1
 1, 7, 139, 229, 425, 461, 725, 1973, 7229, 45859, 47303 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (320*10^n - 23)/9 is prime. Numbers n such that digit 3 followed by n >= 0 occurrences of digit 5 followed by digit 3 is prime. Numbers corresponding to terms <= 1973 are certified primes. For larger numbers see P. De Geest, PDP Reference Table. REFERENCES Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. LINKS Table of n, a(n) for n=1..11. Patrick De Geest, PDP Reference Table - 353. Makoto Kamada, Prime numbers of the form 355...553. Index entries for primes involving repunits. FORMULA a(n) = A082707(n) - 2. EXAMPLE 353 is prime, hence 1 is a term. MATHEMATICA Select[Range[0, 2000], PrimeQ[(320 10^# - 23) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *) PROG (PARI) a=33; for(n=0, 2000, if(isprime(a), print1(n, ", ")); a=10*a+23) (PARI) for(n=0, 2000, if(isprime((320*10^n-23)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A082707. Sequence in context: A171106 A217504 A142295 * A137463 A274525 A221375 Adjacent sequences: A056251 A056252 A056253 * A056255 A056256 A056257 KEYWORD nonn,hard AUTHOR Robert G. Wilson v, Aug 18 2000 EXTENSIONS Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004 Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, May 31 2007, Jun 15 2007 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 Comments section and a link updated by Patrick De Geest, Nov 02 2014 Edited by Ray Chandler, Nov 05 2014 STATUS approved

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Last modified October 2 17:28 EDT 2023. Contains 365837 sequences. (Running on oeis4.)