A101844 Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) + 71 for n > 0.

0, 2, 18, 29, 39, 41, 44, 69, 306, 537, 1608, 1667, 1673, 4491, 4934, 6111, 12818, 14078, 16019, 23004, 50996, 82869

Offset

1,2

Comments

Numbers n such that (350*10^n - 71)/9 is prime.

Numbers n such that digit 3 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.

Numbers corresponding to terms <= 537 are certified primes.

a(23) > 10^5. - Robert Price, May 20 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..22.

Makoto Kamada, Prime numbers of the form 388...881.

Index entries for primes involving repunits.

Formula

a(n) = A102978(n) - 1.

Example

3881 is prime, hence 2 is a term.

Mathematica

#-1&/@(Flatten[Position[NestList[10#+71&, 31, 6200], _?(PrimeQ[#]&)]]) (* Harvey P. Dale, Aug 20 2011 *)

Prog

(PARI) a=31; for(n=0, 1700, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1700, if(isprime((350*10^n-71)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102978.

Sequence in context: A366957 A098561 A141426 * A193598 A190047 A166259

Adjacent sequences: A101841 A101842 A101843 * A101845 A101846 A101847

Keyword

nonn,hard,more

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004

Extensions

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

a(17)-a(20) from Kamada data by Ray Chandler, May 01 2015

a(21)-a(22) from Robert Price, May 20 2015

Status

approved