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 A056250 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 81 for n > 0. 3
 0, 1, 3, 7, 39, 85, 199, 729, 1459, 23671, 28629 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Numbers n such that (180*10^n - 81)/9 is prime. Numbers n such that digit 1 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime. Numbers corresponding to terms <= 1459 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 Patrick De Geest, PDP Reference Table - 191. Makoto Kamada, Prime numbers of the form 199...991. FORMULA a(n) = A082703(n-1) - 2 for n > 1. EXAMPLE 191 is prime, hence 1 is a term. MATHEMATICA Select[Range[0, 2000], PrimeQ[(180 10^# - 81) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *) PROG (PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81) (PARI) for(n=0, 1500, if(isprime((180*10^n-81)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A068649, A082703. Sequence in context: A192198 A061931 A057203 * A113870 A209326 A074582 Adjacent sequences:  A056247 A056248 A056249 * A056251 A056252 A056253 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 28 2004 Edited by N. J. A. Sloane, Jun 15 2007 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 Updated comments section and a link, by Patrick De Geest, Nov 02 2014 Edited by Ray Chandler, Nov 04 2014 STATUS approved

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Last modified July 17 01:41 EDT 2018. Contains 312693 sequences. (Running on oeis4.)