The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A102034 Indices of primes in sequence defined by A(0) = 17, A(n) = 10*A(n-1) + 27 for n > 0. 1
 0, 1, 2, 3, 12, 13, 75, 236, 871, 987, 1279, 1369, 2685, 3611, 4050, 4644, 9161, 10855, 25958, 64881, 144219, 153745, 185929 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Numbers n such that (180*10^n - 27)/9 is prime. Numbers n such that digit 1 followed by n >= 0 occurrences of digit 9 followed by digit 7 is prime. Numbers corresponding to terms <= 987 are certified primes. a(21) > 10^5. - Robert Price, Nov 16 2014 a(24) > 2*10^5. - Robert Price, Jul 11 2015 REFERENCES Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. LINKS Makoto Kamada, Prime numbers of the form 199...997. FORMULA a(n) = A102947(n) - 1. - Robert Price, Nov 16 2014 EXAMPLE 197 is prime, hence 1 is a term. MATHEMATICA Select[Range[0, 1000], PrimeQ[(180 10^# - 27) / 9] &] (* Vincenzo Librandi, Nov 17 2014 *) PROG (PARI) a=17; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+27) (PARI) for(n=0, 1500, if(isprime((180*10^n-27)/9), print1(n, ", "))) (MAGMA) [n: n in [0..500] | IsPrime((180*10^n-27) div 9) ]; // Vincenzo Librandi, Nov 17 2014 CROSSREFS Cf. A000533, A002275, A102947. Sequence in context: A081347 A309176 A074347 * A102150 A039588 A024579 Adjacent sequences:  A102031 A102032 A102033 * A102035 A102036 A102037 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 a(18)-a(20) derived from A102947 by Robert Price, Nov 16 2014 a(21)-a(23) from Robert Price, Jul 11 2015 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified July 30 15:29 EDT 2021. Contains 346359 sequences. (Running on oeis4.)