A101723 Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 21 for n > 0.

0, 1, 3, 6, 9, 11, 19, 23, 29, 41, 61, 187, 303, 339, 714, 803, 1039, 1886, 2078, 2119, 2259, 2422, 3318, 5597, 6071, 22047, 25712, 28643, 43352

Offset

1,3

Comments

Numbers n such that (390*10^n - 21)/9 is prime.

Numbers n such that digit 4 followed by n >= 0 occurrences of digit 3 followed by digit 1 is prime.

a(30) > 10^5. - Robert Price, Mar 30 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..29.

Makoto Kamada, Prime numbers of the form 433...331.

Index entries for primes involving repunits.

Formula

a(n) = A102988(n) - 1.

Example

431 is prime, hence 1 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(390*10^# - 21)/9] &]

Prog

(PARI) a=41; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+21)
(PARI) for(n=0, 1500, if(isprime((390*10^n-21)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102988.

Sequence in context: A329596 A344956 A332369 * A353889 A113944 A198514

Adjacent sequences: A101720 A101721 A101722 * A101724 A101725 A101726

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(26)-a(29) derived from A102988 by Robert Price, Mar 30 2015

Status

approved