A104389 Repdigits which are the average of two successive primes.

4, 6, 9, 99, 111, 4444, 111111111, 555555555555, 9999999999999999, 111111111111111111, 444444444444444444, 555555555555555555555, 777777777777777777777777, 333333333333333333333333333333333, 444444444444444444444444444444444444

Offset

1,1

Links

Michael S. Branicky, Table of n, a(n) for n = 1..31

Formula

a(n) = (A104387(n)+A104388(n))/2.

Prog

(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 r
print(list(islice(agen(), 15))) # Michael S. Branicky, Jun 30 2022

Crossrefs

Corresponding primes: A104387, A104388.

Cf. A054268, A104386, A104387, A104388.

Sequence in context: A317248 A115666 A245044 * A115655 A363248 A084350

Adjacent sequences: A104386 A104387 A104388 * A104390 A104391 A104392

Keyword

nonn,base

Author

Zak Seidov, Mar 04 2005

Extensions

More terms from Giovanni Resta, Apr 05 2006

Status

approved