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A056244 Indices of primes in sequence defined by A(0) = 11, A(n) = 10*A(n-1) + 21 for n > 0. 2
0, 1, 3, 5, 93, 159, 359, 1469, 2897, 3093, 3111, 15697, 17955, 42261, 111031 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Numbers n such that (120*10^n - 21)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.

Numbers corresponding to terms <= 3111 are certified primes. For larger numbers see P. De Geest, PDP Reference Table.

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..15.

Patrick De Geest, PDP Reference Table - 131.

Makoto Kamada, Prime numbers of the form 133...331.

Index entries for primes involving repunits.

FORMULA

a(n) = A082697(n-1) - 2 for n > 1.

EXAMPLE

131 is prime, hence 1 is a term.

MATHEMATICA

Do[If[PrimeQ[(1*10^n + 3*(10^n - 1)/9)*10 + 1], Print[n]], {n, 1, 2500}]

Select[Range[0, 2000], PrimeQ[(120 10^# - 21) / 9] &] (* Vincenzo Librandi, Nov 03 2014 *)

PROG

(PARI) a=11; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+21)

(PARI) for(n=0, 1500, if(isprime((120*10^n-21)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A068645, A082697.

Sequence in context: A271925 A236365 A057663 * A173487 A279310 A103081

Adjacent sequences:  A056241 A056242 A056243 * A056245 A056246 A056247

KEYWORD

nonn,hard,more

AUTHOR

Robert G. Wilson v, Aug 18 2000

EXTENSIONS

More terms and 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

Updates from De Geest site by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(15)=111031 from Ray Chandler, Apr 14 2011

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 25 21:12 EDT 2018. Contains 315425 sequences. (Running on oeis4.)