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 A056244 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 21 for n > 0. 2
 0, 1, 3, 5, 93, 159, 359, 1469, 2897, 3093, 3111, 15697, 17955, 42261, 111031 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Numbers n such that (120*10^n - 21)/9 is prime. Numbers n such that digit 1 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime. Numbers corresponding to terms <= 3111 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 - 131. Makoto Kamada, Prime numbers of the form 133...331. FORMULA a(n) = A082697(n-1) - 2 for n > 1. EXAMPLE 131 is prime, hence 1 is a term. MATHEMATICA Do[If[PrimeQ[(1*10^n + 3*(10^n - 1)/9)*10 + 1], Print[n]], {n, 1, 2500}] Select[Range[0, 2000], PrimeQ[(120 10^# - 21) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *) PROG (PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+21) (PARI) for(n=0, 1500, if(isprime((120*10^n-21)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A068645, A082697. Sequence in context: A271925 A236365 A057663 * A173487 A279310 A103081 Adjacent sequences:  A056241 A056242 A056243 * A056245 A056246 A056247 KEYWORD nonn,hard,more,changed AUTHOR Robert G. Wilson v, Aug 18 2000 EXTENSIONS More terms and 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 Updates from De Geest site by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(15)=111031 from Ray Chandler, Apr 14 2011 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 January 18 04:47 EST 2019. Contains 319269 sequences. (Running on oeis4.)