login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual appeal: Please make a donation to keep the OEIS running! Over 6000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A181539 Smallest number m > 1 such that m^2 == 1 (mod 10^n). 2
9, 49, 249, 1249, 18751, 218751, 781249, 24218751, 74218751, 1425781249, 13574218751, 163574218751, 163574218751, 19836425781249, 19836425781249, 2480163574218751, 12519836425781249, 12519836425781249, 487480163574218751, 15487480163574218751, 215487480163574218751, 215487480163574218751 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) > 10^floor(n/2).

All terms have last digit 1 or 9.

Squares of terms are listed in A085877.

Decimal representation of each term is formed by the reverse concatenation of initial terms of either A063006 or A091661.

Except for 3, there are no solutions for n>1 and m^2 == -1 (mod 10^n). See comment in A063006 under extensions. - Robert G. Wilson v, Jan 26 2013

If a(n)<(10^n)/2 then (10^n-a(n))^2 is also congruent (modulo 10^n), its just not the least. - Robert G. Wilson v, Jan 26 2013

LINKS

Max Alekseyev, Table of n, a(n) for n = 1..1000

EXAMPLE

1249^2 = 1560001 == 1 (mod 10^4), and there is no smaller m > 1 such that m^2 == 1 (mod 10^4). Hence a(4) = 1249.

CROSSREFS

Cf. A085610, A181607. [From R. J. Mathar, Oct 30 2010]

Sequence in context: A192814 A228018 A081655 * A224473 A146798 A055428

Adjacent sequences:  A181536 A181537 A181538 * A181540 A181541 A181542

KEYWORD

nonn

AUTHOR

Kevin Batista (kevin762401(AT)yahoo.com), Oct 29 2010

EXTENSIONS

a(2) through a(4), a(7) through a(11) corrected, comment added, example replaced by Klaus Brockhaus, Nov 01 2010

Edited by N. J. A. Sloane, Oct 29 2010, Nov 09 2010

Constrained the definition to avoid the constant sequence a(n)=1 - R. J. Mathar, Nov 18 2010

a(1) corrected, terms a(13) onward added by Max Alekseyev, Dec 10 2012

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified November 18 09:13 EST 2017. Contains 294879 sequences.