A101128 Indices of primes in sequence defined by A(0) = 71, A(n) = 10*A(n-1) - 9 for n > 0.

0, 1, 2, 3, 4, 7, 8, 44, 135, 141, 157, 242, 922, 1234, 2195, 4649, 6118, 7323, 9542, 13493, 20309, 20359, 232919, 830864, 902707

Offset: 1

Author

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004

Keywords

• nonn
• hard
• more

Comments

Numbers n such that 70*10^n + 1 is prime.

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

Numbers corresponding to terms <= 922 are certified primes.

Examples

			700001 is prime, hence 4 is a term.
		

References

• Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

Links

• Makoto Kamada, Prime numbers of the form 700...001.
• Index entries for primes involving repunits.

Crossrefs

Cf. A000533, A002275, A056804.

Programs

PARI

a=71;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-9)

PARI

for(n=0,1500,if(isprime(70*10^n+1),print1(n,",")))

Formula

a(n) = A056804(n) - 1.

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(21)-a(25) from Kamada data by Ray Chandler, Apr 29 2015