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 A101959 Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) - 23 for n > 0. 1
 2, 8, 14, 20, 26, 86, 179, 1940, 3959, 4994, 8576, 23480, 27854, 80594 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that (220*10^n + 23)/9 is prime. Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime. Numbers corresponding to terms <= 179 are certified primes. With alternating signs, expansion of cosh(atan(x)). a(15) > 10^5. - Robert Price, Mar 15 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 244...447. FORMULA a(n) = A102954(n) - 1. EXAMPLE 2447 is prime, hence 2 is a term. MATHEMATICA Select[Range[0, 100000], PrimeQ[(220*10^# + 23)/9] &] (* Robert Price, Mar 14 2015 *) PROG (PARI) a=27; for(n=0, 2000, if(isprime(a), print1(n, ", ")); a=10*a-23) (PARI) for(n=0, 2000, if(isprime((220*10^n+23)/9), print1(n, ", "))) CROSSREFS Cf. A000533, A002275, A102954. Sequence in context: A082933 A016933 A294960 * A241003 A133229 A161330 Adjacent sequences:  A101956 A101957 A101958 * A101960 A101961 A101962 KEYWORD nonn,hard,more AUTHOR Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 a(12)-a(14) derived from A102954 by Robert Price, Mar 15 2015 STATUS approved

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Last modified January 15 20:47 EST 2019. Contains 319184 sequences. (Running on oeis4.)