A101134 Indices of primes in sequence defined by A(0) = 73, A(n) = 10*A(n-1) - 7 for n > 0.

0, 3, 11, 12, 14, 35, 39, 89, 836, 2423, 2426, 2442, 15924, 16211

Offset

1,2

Comments

Numbers n such that (650*10^n + 7)/9 is prime.

Numbers n such that digit 7 followed by n >= 0 occurrences of digit 2 followed by digit 3 is prime.

Numbers corresponding to terms <= 836 are certified primes.

a(15) > 10^5. - Robert Price, Sep 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..14.

Makoto Kamada, Prime numbers of the form 722...223.

Index entries for primes involving repunits.

Formula

a(n) = A103053(n) - 1.

Example

72223 is prime, hence 3 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(650*10^# + 7)/9] &] (* Robert Price, Sep 22 2015 *)

Prog

(PARI) a=73; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-7)
(PARI) for(n=0, 1000, if(isprime((650*10^n+7)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A103053.

Sequence in context: A365037 A043051 A024545 * A045110 A055046 A141386

Adjacent sequences: A101131 A101132 A101133 * A101135 A101136 A101137

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(13)-a(14) from Kamada data by Ray Chandler, Apr 30 2015

Status

approved