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 A101969 Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 71 for n > 0. 1
 1, 11, 26, 43, 79, 118, 130, 274, 314, 875, 1306, 10894, 17104, 60716 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (260*10^n - 71)/9 is prime. Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime. Numbers corresponding to terms <= 875 are certified primes. a(15) > 10^5. - Robert Price, Mar 16 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 288...881. FORMULA a(n) = A102960(n) - 1. EXAMPLE 281 is prime, hence 1 is a term. MATHEMATICA Select[Range[1000], PrimeQ[(260*10^# - 71)/9] &] (*Robert Price, Mar 17 2015*) PROG (PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71) (PARI) for(n=0, 1500, if(isprime((260*10^n-71)/9), print1(n, ", "))) (Magma) [n: n in [0..350] | IsPrime((260*10^n - 71) div 9)]; // Vincenzo Librandi, Mar 17 2015 CROSSREFS Cf. A000533, A002275, A102960. Sequence in context: A047722 A161451 A100566 * A139576 A166137 A212018 Adjacent sequences: A101966 A101967 A101968 * A101970 A101971 A101972 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004 EXTENSIONS a(12)-a(14) derived from A102960 by Robert Price, Mar 16 2015 STATUS approved

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Last modified December 8 01:12 EST 2022. Contains 358672 sequences. (Running on oeis4.)