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A000689
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Final decimal digit of 2^n.
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10
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1, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6
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OFFSET
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0,2
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COMMENTS
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These are the analogs of the powers of 2 in carryless arithmetic mod 10.
Let G = {2,4,8,6}. Let o be defined as XoY = least significant digit in XY. Then (G,o) is an Abelian group wherein 2 is a generator (Also see the first comment under A001148). - K.V.Iyer, Mar 12 2010
a(n) is also 2^n mod 10.
For n > 0: a(n) = A002081(n) - A002081(n-1).
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LINKS
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Table of n, a(n) for n=0..80.
David Applegate, Marc LeBrun and N. J. A. Sloane, Carryless Arithmetic (I): The Mod 10 Version
Index entries for sequences related to carryless arithmetic
Index entries for sequences related to final digits of numbers
Index entries for linear recurrences with constant coefficients, signature (1, -1, 1).
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FORMULA
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Periodic with period 4.
a(n) = (1/6)*{8*(n mod 4)-[(n+1) mod 4]+2*[(n+2) mod 4]+11*[(n+3) mod 4]}-5*{1-[((n+1)!+1) mod (n+1)]}, with n>=0. - Paolo P. Lava, Jun 25 2007; corrected by Paolo P. Lava, Mar 23 2010
a(n) = +a(n-1) -a(n-2) +a(n-3), n>3. G.f.: (x+3*x^2+5*x^3+1)/((1-x) * (1+x^2)). - R. J. Mathar, Apr 13 2010
a(n) = 5+(1/2)*[(1+3*I)*I^n+(1-3*I)*(-I)^n]-5*[C(2*n,n) mod 2], with n>=0. - Paolo P. Lava, May 10 2010
For n>=1, a(n) = 10 - (4x^3 +47x -27x^2)/3, where x = (n+3) mod 4 + 1.
For n>=1, a(n) = A070402(n) + 5*floor( ((n-1) mod 4)/2 ).
G.f.: 1 / (1 - 2*x / (1 + 5*x^3 / (1 + x / (1 - 3*x / (1 + 3*x))))). - Michael Somos, May 12 2012
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EXAMPLE
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G.f. = 1 + 2*x + 4*x^2 + 8*x^3 + 6*x^4 + 2*x^5 + 4*x^6 + 8*x^7 + 6*x^8 + ...
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MAPLE
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P:=proc(n) local a, i; for i from 0 by 1 to n do a:=1/6*(8*(i mod 4)-((i+1) mod 4)+2*((i+2) mod 4)+11*((i+3) mod 4))-5*(1-(((i+1)!+1) mod (i+1))); print(a); od; end: P(100); # Paolo P. Lava, Jun 25 2007
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MATHEMATICA
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Table[PowerMod[2, n, 10], {n, 0, 200}] (* Vladimir Joseph Stephan Orlovsky, Jun 10 2011 *)
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PROG
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(Sage) [power_mod(2, n, 10)for n in range(0, 81)] # Zerinvary Lajos, Nov 03 2009
(PARI) for(n=0, 80, if(n, {x=(n+3)%4+1; print1(10-(4*x^3+47*x-27*x^2)/3, ", ")}, {print1("1, ")}))
(MAGMA) [2^n mod 10: n in [0..150]]; // Vincenzo Librandi, Apr 12 2011
(Haskell)
a000689 n = a000689_list !! n
a000689_list = 1 : cycle [2, 4, 8, 6] -- Reinhard Zumkeller, Sep 15 2011
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CROSSREFS
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Cf. A173635.
Sequence in context: A125733 A333555 A280426 * A132137 A011180 A103546
Adjacent sequences: A000686 A000687 A000688 * A000690 A000691 A000692
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KEYWORD
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nonn,base,easy
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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