A101959 Indices of primes in sequence defined by A(0) = 27, A(n) = 10*A(n-1) - 23 for n > 0.

2, 8, 14, 20, 26, 86, 179, 1940, 3959, 4994, 8576, 23480, 27854, 80594

Offset

1,1

Comments

Numbers n such that (220*10^n + 23)/9 is prime.

Numbers n such that digit 2 followed by n >= 0 occurrences of digit 4 followed by digit 7 is prime.

Numbers corresponding to terms <= 179 are certified primes.

With alternating signs, expansion of cosh(atan(x)).

a(15) > 10^5. - Robert Price, Mar 15 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 244...447.

Index entries for primes involving repunits.

Formula

a(n) = A102954(n) - 1.

Example

2447 is prime, hence 2 is a term.

Mathematica

Select[Range[0, 100000], PrimeQ[(220*10^# + 23)/9] &] (* Robert Price, Mar 14 2015 *)

Prog

(PARI) a=27; for(n=0, 2000, if(isprime(a), print1(n, ", ")); a=10*a-23)
(PARI) for(n=0, 2000, if(isprime((220*10^n+23)/9), print1(n, ", ")))

Crossrefs

Cf. A000533, A002275, A102954.

Sequence in context: A353466 A016933 A294960 * A241003 A133229 A364615

Adjacent sequences: A101956 A101957 A101958 * A101960 A101961 A101962

Keyword

nonn,hard,more

Author

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

Extensions

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

a(12)-a(14) derived from A102954 by Robert Price, Mar 15 2015

Status

approved