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A104387 Larger of two successive primes the average of which is a repdigit. 4
5, 7, 11, 101, 113, 4447, 111111113, 555555555559, 10000000000000061, 111111111111111131, 444444444444444469, 555555555555555555619, 777777777777777777777787, 333333333333333333333333333333373, 444444444444444444444444444444444497 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Lesser primes A104388, repdigits A104389. What is the next term?
The next term is 444444444444444444444444444444444497. The first term with more than 100 digits is a(22) which has 109 digits. - Harvey P. Dale, Jun 28 2011
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..31
FORMULA
a(n) = prime(A104386(n)+1).
MATHEMATICA
Union[Flatten[Table[NextPrime/@Select[FromDigits/@Table[PadLeft[{i}, n, i], {n, 45}], Mean[{NextPrime[#], NextPrime[#, -1]}]==#&], {i, 9}]]] (* Harvey P. Dale, Jun 28 2011 *)
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 2*r - p
print(list(islice(agen(), 15))) # Michael S. Branicky, Jun 30 2022
CROSSREFS
Sequence in context: A027728 A218275 A086140 * A133761 A057659 A098040
KEYWORD
nonn,base
AUTHOR
Zak Seidov, Mar 04 2005
EXTENSIONS
More terms from Giovanni Resta, Apr 05 2006
STATUS
approved

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Last modified May 21 19:35 EDT 2024. Contains 372738 sequences. (Running on oeis4.)