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 A101848 Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) + 81 for n > 0. 1
 0, 16, 18, 28, 42, 118, 172, 948, 1608, 5578, 19678, 34146, 43492, 97798 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that (360*10^n - 81)/9 is prime. Numbers n such that digit 3 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime. Numbers corresponding to terms <= 948 are certified primes. a(15) > 10^5. - Robert Price, Mar 17 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 399...991. FORMULA a(n) = A101396(n) - 1. EXAMPLE 31 is prime, hence 0 is a term. MATHEMATICA Select[Range[0, 1000], PrimeQ[(360*10^# - 81)/9] &] (* Robert Price, Mar 17 2015 *) PROG (PARI) a=31; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81) (PARI) for(n=0, 1500, if(isprime((360*10^n-81)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A101396. Sequence in context: A057369 A178110 A117100 * A032401 A223588 A183401 Adjacent sequences:  A101845 A101846 A101847 * A101849 A101850 A101851 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 a(12)-a(14) derived from A101396 by Robert Price, Mar 17 2015 STATUS approved

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