A104389 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

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