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URL: https://oeis.org/A215730

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A215730
a(n) is the smallest m for which 7^m contains n consecutive identical digits.
14
0, 6, 31, 71, 172, 175, 1961, 6176, 33836, 61282, 305871, 856635, 2135396, 7291510, 11032874, 30775389
OFFSET
1,2
COMMENTS
a(13) > 1116000. - Chai Wah Wu, Dec 17 2014
a(14) > 7*10^6. - Giovanni Resta, Apr 20 2016
EXAMPLE
7^31 = 157775382034845806615042743 contains 3 consecutive identical digits.
PROG
(Python)
import sys
sys.set_int_max_str_digits(200000)
def a(n):
st = "0123456789"
for k in range(10**6):
s = str(7**k)
tot = 0
for i in st:
if s.count(i*n) > 0:
tot += 1
break
if tot > 0:
return k
n = 1
while n < 10:
print(a(n), end=', ')
n += 1
# Derek Orr, Jul 28 2014
(Python)
def A215730(n):
l, x = [str(d)*n for d in range(10)], 1
for m in range(10**9):
s = str(x)
for k in l:
if k in s:
return m
x *= 7
return 'search limit reached'
# Chai Wah Wu, Dec 17 2014
CROSSREFS
Sequence in context: A254502 A025524 A042841 * A187508 A087725 A273790
KEYWORD
nonn,base,hard
AUTHOR
V. Raman, Aug 22 2012
EXTENSIONS
Added a(10), Rick van der Hoorn, Mar 26 2013
a(11)-a(12) from Hiroaki Yamanouchi, Aug 29 2014
a(13) from Giovanni Resta, Apr 19 2016
a(14)-a(15) from Bert Dobbelaere, Feb 15 2019
a(16) from Paul Geneau de Lamarlière, Jul 16 2024
STATUS
approved