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A056248 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 61 for n > 0. 1
0, 5, 47, 101, 191, 365, 1001, 20363, 37445, 56081 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that (160*10^n - 61)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.

Numbers corresponding to terms <= 1001 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..10.

Patrick De Geest, PDP Reference Table - 171.

Makoto Kamada, Prime numbers of the form 177...771.

Index entries for primes involving repunits.

FORMULA

a(n) = A082701(n-1) - 2 for n > 1.

EXAMPLE

1777771 is prime, hence 5 is a term.

MATHEMATICA

Flatten[Position[NestList[10#+61&, 11, 38000], _?PrimeQ]-1] (* Harvey P. Dale, Apr 17 2014 *)

Select[Range[0, 2000], PrimeQ[(160 10^# - 61) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)

PROG

(PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+61)

(PARI) for(n=0, 1500, if(isprime((160*10^n-61)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A082701.

Sequence in context: A196160 A136088 A141890 * A126575 A229613 A277607

Adjacent sequences:  A056245 A056246 A056247 * A056249 A056250 A056251

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

Added one more term from PDP table and a link. Updated comments section and 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 21 23:49 EDT 2018. Contains 315266 sequences. (Running on oeis4.)