A101966 Indices of primes in sequence defined by A(0) = 21, A(n) = 10*A(n-1) + 61 for n > 0.

1, 5, 32, 68, 149, 935, 3134, 5837, 6989, 20785, 57137

Offset

1,2

Comments

Numbers n such that (250*10^n - 61)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 7 followed by digit 1 is prime.

Numbers corresponding to terms <= 935 are certified primes.

a(12) > 10^5. - Robert Price, Feb 27 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 277...771.

Index entries for primes involving repunits.

Formula

a(n) = A098960(n) - 1.

Example

271 is prime, hence 1 is a term.

Mathematica

Do[If[PrimeQ[(250*10^n-61)/9], Print[n]], {n, 1, 3250}] (* Stefan Steinerberger, Jan 31 2006 *)

Prog

(PARI) a=21; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+61)
(PARI) for(n=0, 1500, if(isprime((250*10^n-61)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A098960.

Sequence in context: A350997 A073694 A322952 * A341423 A184536 A363411

Adjacent sequences: A101963 A101964 A101965 * A101967 A101968 A101969

Keyword

nonn,hard,more

Author

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

Extensions

a(7) from Stefan Steinerberger, Jan 31 2006

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

a(10)-a(11) from Robert Price, Feb 27 2015

Status

approved