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 A101715 Indices of primes in sequence defined by A(0) = 49, A(n) = 10*A(n-1) - 81 for n > 0. 1
 1, 3, 4, 7, 8, 27, 190, 195, 2037, 34413, 39265, 50578, 94285, 108411, 130479, 178090, 185354 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that 40*10^n + 9 is prime. Numbers n such that digit 4 followed by n >= 0 occurrences of digit 0 followed by digit 9 is prime. Numbers corresponding to terms <= 195 are certified primes. a(18) > 2*10^5. - Robert Price, May 24 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 400...009. FORMULA a(n) = A101394(n+1) - 1. EXAMPLE 40009 is prime, hence 3 is a term. MATHEMATICA Select[Range[0, 1000], PrimeQ[40*10^# + 9] &] (* Robert Price, May 24 2015 *) PROG (PARI) a=49; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-81) (PARI) for(n=0, 1500, if(isprime(40*10^n+9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A101394. Sequence in context: A214096 A169943 A215110 * A249751 A169893 A169899 Adjacent sequences:  A101712 A101713 A101714 * A101716 A101717 A101718 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004 EXTENSIONS 2037 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 a(10)-a(11) from Kamada data by Ray Chandler, Apr 30 2015 a(12)-a(17) from Robert Price, May 24 2015 STATUS approved

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Last modified April 5 22:12 EDT 2020. Contains 333260 sequences. (Running on oeis4.)