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A102025 Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 21 for n > 0. 1
0, 2, 4, 6, 10, 11, 27, 35, 383, 628, 719, 1397, 2001, 2621, 6456, 19304, 38507, 46454, 95975 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that (150*10^n + 21)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.

Numbers corresponding to terms <= 719 are certified primes.

a(20) > 10^5. - Robert Price, Nov 15 2014

REFERENCES

Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

LINKS

Table of n, a(n) for n=1..19.

Makoto Kamada, Prime numbers of the form 166...669.

Index entries for primes involving repunits

FORMULA

a(n) = A102941(n) - 1. - Robert Price, Nov 15 2014

EXAMPLE

1669 is prime, hence 2 is a term.

PROG

(PARI) a=19; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-21)

(PARI) for(n=0, 1500, if(isprime((150*10^n+21)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A102941.

Sequence in context: A014666 A067852 A253968 * A309374 A226295 A090127

Adjacent sequences:  A102022 A102023 A102024 * A102026 A102027 A102028

KEYWORD

nonn,hard,more

AUTHOR

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004

EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(16)-a(19) derived from A102941 by Robert Price, Nov 15 2014

STATUS

approved

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Last modified June 5 09:58 EDT 2020. Contains 334840 sequences. (Running on oeis4.)