A101150 Indices of primes in sequence defined by A(0) = 79, A(n) = 10*A(n-1) - 21 for n > 0.

0, 1, 2, 5, 7, 17, 18, 20, 31, 47, 64, 71, 98, 119, 200, 307, 716, 967, 1343, 2131, 3566, 3967, 8326, 10597, 12464, 15874, 18894, 28610, 29417, 83692

Offset

1,3

Comments

Numbers n such that (690*10^n + 21)/9 is prime.

Numbers n such that digit 7 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.

Numbers corresponding to terms <= 967 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..30.

Makoto Kamada, Prime numbers of the form 766...669.

Index entries for primes involving repunits.

Formula

a(n) = A103065(n) - 1.

Example

7669 is prime, hence 2 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(690*10^# + 21)/9] &] (* Robert Price, Oct 08 2015 *)

Prog

(PARI) a=79; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-21)
(PARI) for(n=0, 1500, if(isprime((690*10^n+21)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103065.

Sequence in context: A281794 A193462 A306636 * A276322 A337986 A174281

Adjacent sequences: A101147 A101148 A101149 * A101151 A101152 A101153

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(28)-a(29) from Kamada data by Ray Chandler, Apr 30 2015

a(30) from Robert Price, Oct 08 2015

Status

approved