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A056249 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 71 for n > 0. 2
0, 1, 7, 13, 39, 91, 127, 883, 9423, 14767, 19257, 31233 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Numbers n such that (170*10^n - 71)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.

Numbers corresponding to terms <= 883 are certified primes.

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..12.

Patrick De Geest, PDP Reference Table - 181.

Makoto Kamada, Prime numbers of the form 188...881.

Index entries for primes involving repunits.

FORMULA

a(n) = A082702(n-1) - 2 for n > 1.

EXAMPLE

181 is prime, hence 1 is a term.

MATHEMATICA

Select[Range[0, 2000], PrimeQ[(170 10^# - 71) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)

PROG

(PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71)

(PARI) for(n=0, 1500, if(isprime((170*10^n-71)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A068648, A082702.

Sequence in context: A144729 A123250 A062591 * A107207 A045464 A134854

Adjacent sequences:  A056246 A056247 A056248 * A056250 A056251 A056252

KEYWORD

nonn,hard,more

AUTHOR

Robert G. Wilson v, Aug 18 2000

EXTENSIONS

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

Edited by N. J. A. Sloane, Jun 15 2007

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

Updated and added a link, by Patrick De Geest, Nov 02 2014

Edited by Ray Chandler, Nov 04 2014

STATUS

approved

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Last modified September 25 11:05 EDT 2018. Contains 315389 sequences. (Running on oeis4.)