A101972 Indices of primes in sequence defined by A(0) = 29, A(n) = 10*A(n-1) - 1 for n > 0.

0, 6, 7, 10, 13, 18, 31, 40, 52, 60, 132, 156, 238, 1236, 30253, 34422, 139288

Offset

1,2

Comments

Numbers n such that (260*10^n + 1)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.

Numbers corresponding to terms <= 238 are certified primes.

a(18) > 2*10^5. - Robert Price, Jan 29 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..17.

Makoto Kamada, Prime numbers of the form 288...889.

Index entries for primes involving repunits.

Formula

a(n) = A102963(n+1) - 1.

Example

28888889 is prime, hence 6 is a term.

Mathematica

Flatten[Position[Table[FromDigits[Join[PadRight[{2}, n, 8], {9}]], {n, 1300}], _?PrimeQ]]-1 (* Harvey P. Dale, Sep 22 2012 *)

Prog

(PARI) a=29; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-1)
(PARI) for(n=0, 1500, if(isprime((260*10^n+1)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102963.

Sequence in context: A135121 A309854 A083133 * A102727 A315831 A287386

Adjacent sequences: A101969 A101970 A101971 * A101973 A101974 A101975

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004

Extensions

a(15)-a(17) derived from A102963 by Robert Price, Jan 29 2015

Status

approved