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

1,2

COMMENTS

a(13) > 1116000. - Chai Wah Wu, Dec 17 2014

a(14) > 7*10^6. - Giovanni Resta, Apr 20 2016

LINKS

Table of n, a(n) for n=1..15.

EXAMPLE

7^31 = 157775382034845806615042743 contains 3 consecutive identical digits.

PROG

(Python)

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

Cf. A215736, A045875.

Sequence in context: A254502 A025524 A042841 * A187508 A087725 A273790

Adjacent sequences:  A215727 A215728 A215729 * A215731 A215732 A215733

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

STATUS

approved

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Last modified May 18 23:31 EDT 2022. Contains 353826 sequences. (Running on oeis4.)