A101961 Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 41 for n > 0.

1, 2, 4, 5, 11, 185, 434, 1745, 3446, 18797, 70208

Offset

1,2

Comments

Numbers n such that (230*10^n - 41)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.

Numbers corresponding to terms <= 434 are certified primes.

a(12) > 10^5. - Robert Price, Mar 04 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..11.

Makoto Kamada, Prime numbers of the form 255...551.

Index entries for primes involving repunits.

Formula

a(n) = A098930(n) - 1.

Example

255551 is prime, hence 4 is a term.

Prog

(PARI) a=21; for(n=0, 1800, if(isprime(a), print1(n, ", ")); a=10*a+41)
(PARI) for(n=0, 1800, if(isprime((230*10^n-41)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A098930.

Sequence in context: A279050 A317828 A293274 * A285055 A344712 A351753

Adjacent sequences: A101958 A101959 A101960 * A101962 A101963 A101964

Keyword

nonn,hard,more

Author

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

Extensions

3446 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(10)-a(11) from Robert Price, Mar 04 2015

Status

approved