A101147 Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) + 51 for n > 0.

0, 1, 5, 24, 68, 252, 299, 383, 433, 720, 821, 882, 2191, 2648, 3440, 3444, 8027, 11654

Offset

1,3

Comments

Numbers n such that (690*10^n - 51)/9 is prime.

Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 1 is prime.

Numbers corresponding to terms <= 882 are certified primes.

a(19) > 10^5. - Robert Price, Oct 22 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..18.

Makoto Kamada, Prime numbers of the form 766...661.

Index entries for primes involving repunits.

Formula

a(n) = A103063(n+1) - 1.

Example

761 is prime, hence 1 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(690*10^# - 51)/9] &] (* Robert Price, Oct 22 2015 *)

Prog

(PARI) a=71; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a+51)
(PARI) for(n=0, 1000, if(isprime((690*10^n-51)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103063.

Sequence in context: A135703 A258290 A205669 * A274723 A006328 A390534

Adjacent sequences: A101144 A101145 A101146 * A101148 A101149 A101150

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18) from Kamada data by Ray Chandler, Apr 30 2015

Status

approved