A216479 - OEIS

A216479

a(n) is the least multiple of n which uses only the digit 1, or a(n) = -1 if no such multiple exists.

1, -1, 111, -1, -1, -1, 111111, -1, 111111111, -1, 11, -1, 111111, -1, -1, -1, 1111111111111111, -1, 111111111111111111, -1, 111111, -1, 1111111111111111111111, -1, -1, -1, 111111111111111111111111111, -1, 1111111111111111111111111111, -1, 111111111111111, -1, 111111, -1, -1, -1, 111, -1, 111111, -1, 11111, -1

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OFFSET

1,3

COMMENTS

a(n) = -1 if and only if n is a multiple of 2 or 5. See comment in A216485. - Chai Wah Wu, Jun 21 2015

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..1000

MATHEMATICA

Array[Which[GCD[#, 10] != 1, -1, Mod[#, 3] == 0, Block[{k = 1}, While[Mod[k, #] != 0, k = 10 k + 1]; k], True, (10^MultiplicativeOrder[10, #] - 1)/9] &, 42] (* Michael De Vlieger, Dec 11 2020 *)

PROG

(Python)

def A216479(n):

if n % 2 == 0 or n % 5 == 0:

return -1

rem = 1

while rem % n != 0:

rem = rem*10 + 1

return rem

# Azanul Haque, Nov 28 2020

CROSSREFS

Cf. A084681 (number of 1's), A190301 (multiplier).

Cf. A004290, A079339, A181060, A181061.

Sequence in context: A280142 A077573 A266435 * A123698 A123727 A282915

Adjacent sequences: A216476 A216477 A216478 * A216480 A216481 A216482

KEYWORD

sign,base

AUTHOR

V. Raman, Sep 07 2012

STATUS

approved