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 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

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