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 A101540 Indices of primes in sequence defined by A(0) = 69, A(n) = 10*A(n-1) - 1 for n > 0. 1
 4, 56, 500, 514, 626, 640, 724, 53110, 65330, 109672 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that (620*10^n + 1)/9 is prime. Numbers n such that digit 6 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime. Numbers corresponding to terms <= 724 are certified primes. 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 688...889. FORMULA a(n) = A103047(n) - 1. EXAMPLE 688889 is prime, hence 4 is a term. PROG (PARI) a=69; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-1) (PARI) for(n=0, 1000, if(isprime((620*10^n+1)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A103047. Sequence in context: A204108 A077122 A020540 * A224181 A026740 A191466 Adjacent sequences:  A101537 A101538 A101539 * A101541 A101542 A101543 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004 EXTENSIONS a(8)-a(10) from Kamada data by Ray Chandler, Apr 30 2015 STATUS approved

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Last modified August 17 22:32 EDT 2018. Contains 313817 sequences. (Running on oeis4.)