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 A101721 Indices of primes in sequence defined by A(0) = 47, A(n) = 10*A(n-1) - 43 for n > 0. 1
 0, 3, 135, 180, 324, 3291, 3870, 4689, 7875, 11910, 16095, 21648, 43044 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (380*10^n + 43)/9 is prime. Numbers n such that digit 4 followed by n >= 0 occurrences of digit 2 followed by digit 7 is prime. Numbers corresponding to terms <= 324 are certified primes. a(14) > 10^5. - Robert Price, May 08 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 422...227. FORMULA a(n) = A102986(n) - 1. EXAMPLE 42227 is prime, hence 3 is a term. MATHEMATICA Select[Range[0, 300], PrimeQ[(380*10^# + 43)/9] &] (* Robert Price, May 08 2015 *) PROG (PARI) a=47; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-43) (PARI) for(n=0, 1500, if(isprime((380*10^n+43)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A102986. Sequence in context: A239426 A157086 A051376 * A173582 A065973 A110973 Adjacent sequences:  A101718 A101719 A101720 * A101722 A101723 A101724 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 a(10)-a(13) from Kamada data by Ray Chandler, Apr 30 2015 STATUS approved

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Last modified August 8 08:27 EDT 2020. Contains 336293 sequences. (Running on oeis4.)