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!)
 A101580 Indices of primes in sequence defined by A(0) = 57, A(n) = 10*A(n-1) - 23 for n > 0. 1
 1, 5, 7, 17, 52, 67, 241, 275, 319, 563, 619, 1781, 2339, 2453, 5764, 15529, 15604, 22937 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (490*10^n + 23)/9 is prime. Numbers n such that digit 5 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime. a(19) > 10^5. - Robert Price, Jul 29 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 544...447. FORMULA a(n) = A103015(n) - 1. EXAMPLE 547 is prime, hence 1 is a term. MATHEMATICA Select[Range[0, 100000], PrimeQ[(490*10^# + 23)/9] &] (* Robert Price, Ju1 29 2015 *) PROG (PARI) a=57; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-23) (PARI) for(n=0, 1000, if(isprime((490*10^n+23)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A103015. Sequence in context: A019404 A248085 A079604 * A190663 A123195 A248480 Adjacent sequences:  A101577 A101578 A101579 * A101581 A101582 A101583 KEYWORD nonn,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 09 2004 EXTENSIONS Three additional terms, corresponding to probable primes, from Ryan Propper, Jun 18 2005 5764 from Herman Jamke (hermanjamke(AT)fastmail.fm), Apr 28 2007 Edited by T. D. Noe, Oct 30 2008 a(16)-a(18) by Ray Chandler, Apr 24 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 February 18 21:20 EST 2020. Contains 332028 sequences. (Running on oeis4.)