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A101719 Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 11 for n > 0. 1
0, 1, 3, 6, 21, 22, 79, 4426, 31318, 80149, 91143 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Numbers n such that (380*10^n - 11)/9 is prime.

Numbers n such that digit 4 followed by n >= 0 occurrences of digit 2 followed by digit 1 is prime.

Numbers corresponding to terms <= 79 are certified primes.

a(12) > 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

Table of n, a(n) for n=1..11.

Makoto Kamada, Prime numbers of the form 422...221.

Index entries for primes involving repunits.

FORMULA

a(n) = A102984(n) - 1.

EXAMPLE

421 is prime, hence 1 is a term.

MATHEMATICA

Select[Range[0, 3000], PrimeQ[(380*10^# - 11)/9] &] (* Robert Price, May 08 2015 *)

PROG

(PARI) a=41; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+11)

(PARI) for(n=0, 1500, if(isprime((380*10^n-11)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A102984.

Sequence in context: A328567 A064366 A260157 * A173165 A210504 A101582

Adjacent sequences:  A101716 A101717 A101718 * A101720 A101721 A101722

KEYWORD

nonn,hard,more

AUTHOR

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004

EXTENSIONS

4426 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(9) from Erik Branger May 01 2013 by Ray Chandler, Apr 30 2015

a(10)-a(11) from Robert Price, May 08 2015

STATUS

approved

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Last modified December 12 09:36 EST 2019. Contains 329953 sequences. (Running on oeis4.)