A104388 Lesser of two successive primes the average of which is a repdigit.

3, 5, 7, 97, 109, 4441, 111111109, 555555555551, 9999999999999937, 111111111111111091, 444444444444444419, 555555555555555555491, 777777777777777777777767, 333333333333333333333333333333293, 444444444444444444444444444444444391

Offset: 1

Author

Zak Seidov, Mar 04 2005

Keywords

• nonn
• base

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..31

Crossrefs

Cf. A054268, A104386, A104387 (larger primes), A104389 (repdigits).

Programs

Python

from itertools import count, islice
from sympy import isprime, prevprime
def agen():
    for d in count(1):
        ru = int("1"*d)
        for r in range(ru, 10*ru, ru):
            if r > 2:
                p = prevprime(r)
                if isprime(r + (r-p)) and prevprime(r+(r-p)) == p:
                    yield p
print(list(islice(agen(), 15))) # Michael S. Branicky, Jun 30 2022

Formula

a(n) = prime(A104386(n)).

Extensions

More terms from Giovanni Resta, Feb 09 2006