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 A008611 a(n) = a(n-3) + 1, with a(0)=a(2)=1, a(1)=0. 40
 1, 0, 1, 2, 1, 2, 3, 2, 3, 4, 3, 4, 5, 4, 5, 6, 5, 6, 7, 6, 7, 8, 7, 8, 9, 8, 9, 10, 9, 10, 11, 10, 11, 12, 11, 12, 13, 12, 13, 14, 13, 14, 15, 14, 15, 16, 15, 16, 17, 16, 17, 18, 17, 18, 19, 18, 19, 20, 19, 20, 21, 20, 21, 22, 21, 22, 23, 22, 23, 24, 23, 24, 25, 24, 25, 26, 25, 26, 27, 26, 27, 28 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Molien series of 2-dimensional representation of cyclic group of order 3 over GF(2). One step back, two steps forward. The crossing number of the graph C(n, {1,3}), n >= 8, is [n/3] + n mod 3, which gives this sequence starting at the first 4. [Yang Yuansheng et al.] A Chebyshev transform of A078008. The g.f. is the image of (1-x)/(1-x-2x^2) (g.f. of A078008) under the Chebyshev transform A(x)-> 1/(1+x^2))A(x/(1+x^2)). - Paul Barry, Oct 15 2004 A047878 is an essentially identical sequence. - Anton Chupin, Oct 24 2009 Rhyme scheme of Dante Alighieri's "Divine Comedy." - David Gaita, Feb 11 2011 A194960 results from deleting the first four terms of A008611. Note that deleting the first term or first four terms of A008611 leaves a concatenation of segments (n, n+1, n+2); for related concatenations, see   A008619, (n,n+1) after deletion of first term;   A053737, (n,n+1,n+2,n+3) beginning with n=0;   A053824, (n to n+4) beginning with n=0. - Clark Kimberling, Sep 07 2011 It appears that a(n) is the number of roots of x^(n+1) + x + 1 inside the unit circle. - Michel Lagneau, Nov 02 2012 Also apparently for n >= 2: a(n) is the largest remainder r that results from dividing n+2 by 1..n+2 more than once, i.e., a(n) = max(i, A072528(n+2,i)>1). - Ralf Stephan, Oct 21 2013 Number of n-element subsets of [n+1] whose sum is a multiple of 3. a(4) = 1: {1,2,4,5}. - Alois P. Heinz, Feb 06 2017 It appears that a(n) is the number of roots of the Fibonacci polynomial F(n+2,x) strictly inside the unit circle of the complex plane. - Michel Lagneau, Apr 07 2017 For the proof of the preceding conjecture see my comments under A008615 and A049310. Chebyshev S(n,x) = i^n*F(n+1,-i*x), with i = sqrt(-1). - Wolfdieter Lang, May 06 2017 REFERENCES D. J. Benson, Polynomial Invariants of Finite Groups, Cambridge, 1993, p. 103. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 447 G. P. Michon, Counting Polyhedra Yang Yuansheng et al., The crossing number of C(n; {1,3}), Discr. Math. 289 (2004), 107-118. Index entries for linear recurrences with constant coefficients, signature (1,0,1,-1). FORMULA a(n) = a(n-3) + 1 = (n-1) - 2*floor((n-1)/3). G.f.: (1 + x^2 + x^4)/(1 - x^3)^2. After the initial term, has form {n, n+1, n+2} for n=0, 1, 2, ... From Paul Barry, Mar 18 2004: (Start) a(n) = Sum_{k=0..n} (-1)^floor(2*(k-2)/3); a(n) = 4*sqrt(3)*cos(2*Pi*n/3 + Pi/6)/9 + (n+1)/3. (End) From Paul Barry, Oct 15 2004: (Start) G.f.: (1-x+x^2)/( (1+x+x^2)*(x-1)^2); a(n) = Sum_{k=0..floor(n/2)} binomial(n-k, k)*A078008(n-2k)*(-1)^k. (End) a(n) = -a(-2-n) for all n in Z. Euler transform of length 6 sequence [ 0, 1, 2, 0, 0, -1]. - Michael Somos, Jan 23 2014 a(n) = ((n-1) mod 3) + floor((n-1)/3). - Wesley Ivan Hurt, May 18 2014 PSUM transform of A257075. - Michael Somos, Apr 15 2015 a(n) = A194960(n-3), n >= 0, with extended A194960. See the a(n) formula two lines above. - Wolfdieter Lang, May 06 2017 EXAMPLE G.f. = 1 + x^2 + 2*x^3 + x^4 + 2*x^5 + 3*x^6 + 2*x^7 + 3*x^8 + 4*x^9 + ... MAPLE with(numtheory): for n from 1 to 70 do:it:=0: y:=[fsolve(x^n+x+1, x, complex)] : for m from 1 to nops(y) do : if abs(y[m])< 1 then it:=it+1:else fi:od: printf(`%d, `, it):od: A008611:=n->(n-1)-2*floor((n-1)/3); seq(A008611(n), n=0..50); # Wesley Ivan Hurt, May 18 2014 MATHEMATICA With[{nn=30}, Riffle[Riffle[Range[nn], Range[0, nn-1]], Range[nn], 3]] (* or *) RecurrenceTable[{a==a==1, a==0, a[n]==a[n-3]+1}, a, {n, 90}] (* Harvey P. Dale, Nov 06 2011 *) LinearRecurrence[{1, 0, 1, -1}, {1, 0, 1, 2}, 100] (* Vladimir Joseph Stephan Orlovsky, Feb 23 2012 *) a[ n_] := Quotient[n - 1, 3] + Mod[n + 2, 3]; (* Michael Somos, Jan 23 2014 *) PROG (MAGMA) [(n-1)-2*Floor((n-1)/3): n in [0..90]]; // Vincenzo Librandi, Aug 21 2011 (Haskell) a008611 n = n' + mod r 2 where (n', r) = divMod (n + 1) 3 a008611_list = f [1, 0, 1] where f xs = xs ++ f (map (+ 1) xs) -- Reinhard Zumkeller, Nov 25 2013 (PARI) {a(n) = (n-1) \ 3 + (n+2) % 3}; /* Michael Somos, Jan 23 2014 */ CROSSREFS Cf. A058207, A058788, A194960, A257075. Sequence in context: A246017 A116939 A253174 * A025798 A161064 A070086 Adjacent sequences:  A008608 A008609 A008610 * A008612 A008613 A008614 KEYWORD nonn,easy,nice AUTHOR N. J. A. Sloane, Mar 15 1996 STATUS approved

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Last modified December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)